I want to play through the game once as each of the 10 races and focus on different archetypes and questlines each time.

I made the mistake in my first ES game, Oblivion, of making my first character complete all the questlines possible in about 85 hours. Making a new character wasn't all that interesting because I'd already seen everything unique, and the only new things were repetitive dungeons. Next I played Morrowind and purposely did only the main quest with my first character, along with some side quests and one guild questline. My next characters were much more fun because there were whole questlines (like the legion, the house factions, other guilds) that I hadn't touched. To this day I still haven't seen all of the Morrowind questlines, and I can always return to it and find some entirely new content.

Anyway, here is my plan for Skyrim based on what I know (can't get the game till this friday because I live in the remote caribbean... .

As far as I know there are questlines for the following: The Main Quest, Thieves Guild, Companions Guild, Imperial Faction, Rebel Faction, and Dark Brotherhood.
Then there are secondary experiences that can round out and change those characters: Werewolves, Vampires, being a devout follower of a religion, Collecting books and lore, Collecting rare artifacts and baubles, Owning and decorating a home, being a hunter, alchemy, crafting weapons and armor.

My characters I plan to play are an Imperial Battlemage who does the MQ and Mages guild only, a Nord Warrior (sword and shield) who does the MQ and Stormcloaks, a Khajit Female thief who sides with the empire and works in the thieves guild, an Argonian Nightblade who uses stealth and magic to assassinate foes through the Dark Brotherhood and also just is evil and murders people, an Orc "Brawler" who uses a heavy warhammer and heavy armor while completing the Main Quest and the Companions guild, A High Elf pure mage who helps the rebels and completes the mage guild, a Dunmer Necromancer who is evil, a vampire and joins the Dark Brotherhood, A Breton Female Summoner who helps the Imperials and completes the main quest and mages guild, A Wood Elf Archer who is a thief, a companion, and a werewolf, and lastly, A Red Guard "swashbuckler" who is a thief a rebel and a companion.

I consider a quest line to be anything more than 10 hours of content to complete with no fast travel. I will also be limiting myself to an hour and a half of playtime per day (had my wife enter in the code for the "family timer" to keep me to it), so the first 3 characters alone should last me months (It's really hard to get games out here so this game has to last me). I figured if the first three take me a few months, there should be some DLC available to give some of the ones farther down the list something new to try.

Sorry this is so long, but I wanted to know if I'm missing a questline that would last 10 hours or more, or an interesting secondary activity (such as collecting all the books in the game) that I could give to some of the characters who have less new quest content to work with. Also is anyone else planning out their playthroughs and potentially restricting their time per day to get the most time out of the game? Also I'm on Xbox so mods aren't an option. No spoilers please, just let me know if there are some other quest branches I could do, especially if for example the mage guild can branch in two ways or something.

TLDR: What are some other questline type activities that can keep each playthrough fresh for 10 characters on an Xbox 360.

Nobody?

I think you've kinda said it all.

I really like your way of playing, I should have done the same

There is such a ton of things to do, so I guess neither of your characters will have a lack of work to do.

I already made myself a second character, a female hunter. I'm not doing any quests with her so far, just running around and hunting, tryin to make a living lol.
Its a funny feeling when she meets a dragon (I glitched my way to level 100 sneaking) because she is soooo powerless against it, so its just run for your life lol.

I'm doing something like that, my first playthrough is the most logical one playing a good Nord following the main quest, the companions, maybee stormcloacks and some secondary quests when I go to towns.
On the second I will be a more wicked character and will do the thieves guild, black brotherhood and whatever it comes.

What I love about this game is that just by playing your initial plan changes because you simply follow the most natural way to you so i don't plan a whole lot.

Yeah that's what I'm hoping to accomodate for, I'm sure my plans will change as I discover things, but I'm hoping to set in a few hard and fast rules to keep myself from gobbling up all the big stuff on my first few runs through or else I won't have something to look forward to. I know most people put the MQ off a bit but in my case I really want to see what's going to happen. I'll take my time getting to it though with that first character, I'm roleplaying a sword and staff imperial who is fed up with the runaround they are giving him in Cyrodill about magic, he wants to go to Winterhold to really learn magic, but gets caught up in the mess at the border.

I'll probably make my way to winterhold right after I get out of the tutorial dungeon.

To the original poster, here is a small gift for you. I, like you, want to play the game several times, with different... let's call them "personalities". I will try to play my role as best as possible: with a rowdy barbarian, no lockpicking or sneaking; with an assassin, no charging forward; with a shameless opportunist, no selfless acts; and so forth. Here are some backstories that I just made up, to help you "live" your character better. Of course, these are MY stories, meaning that they don't necessarily have the same effect on you. Feel free to change them as much as you need for your own characters, or even better, to write your own background stories! This post is only to show you how I do it: here is a few examples.

Character n.1: Publius Valerius Corvinus, Imperial

Tomorrow I will cross the border and enter Skyrim. Apparently, the region is in the middle of a civil war, with the Empire on one side and some Nord rebels on the other. I don't even know what they are fighting about, but it's no matter. One thing I've learnt is that where is a war, there also are plenty of chances to become rich. And, honestly, I am tired of "just surviving"; I've tried being an honest merchant in Cyrodiil for a while, but it's always the same. The rich, the powerful, even the bloody legion often take advantage of their position to get what they want from you. Hopefully, things will be different in Skyrim. In any way... it's up to me to make my own fortune. I hope I will find a way to do it without becoming one of those greedy bastards that I'm running away from.
* Note to self: I will have to learn how to fight. I am not too fond of danger, but I'm in a place full of wild beasts, bandits, and factions trying to kill each other. I will not always be able to avoid battle; I will have to defend myself by all means necessary. May Akatosh give me strength.

Character n.2 : Aethelric, Nord

Aethelric rested, on the road to his homeland,
Travel-weary and battle-stained. Forth he had ventured
For more than a forthnight, from his home in Bruma,
To fight with his folk, and free from the Imperial
His long-loved land, the snow-blessed Skyrim.
With five loyal liegemen, willing and worthy warriors,
From Bruma he fared forth, at that fated moment.
But bandits they met, three times their number;
From the fierce battle alone he had lived.
To Skyrim he rides, his cold mother calling
To battle and blood, to fortune and fame
To strife with the Stormcloaks, feeders of ravens,
His heavy heart murmuring, Skyrim or death!

Character n.3: Sedaves Hlerayn, dunmer.

Bloody idiots, the Nords. Always charging forward to battle, thinking of glory and plunder, with nary a thought given to their own survival. Personally, I've found that you never have to look for battle too hard enough, because sooner or later battle finds you. And when it finds me, I am always ready. Leave fighting to Orcs and Nords: me, I kill. As efficiently and, may I say, soberly as possible. I never kill for the sake of it, or for a few coins. Strange as it may sound from the lips of one who just killed three men in cold blood, I respect life. But if someone makes himself too big a nuisance, or if I know that someone is after my skin, well, I am not going to challenge him to a duel. I am going to kill him, it's that simple. An arrow in the back or a knife to the throat work wonders against someone who has deserved them, and they are not less honorable than challenging someone to a swordfight to defend the honor of your clan or to prove that you're the strongest. If anything, it's slightly less stupid, and much more effective. I have never entered a battle I was not prepared for. When I start to fight, it means I have already won. Going to battle unprepared is the shortest route for a trip to the graveyard, and no thanks, I am not interested in joining my ancestors yet. When I have to fight, most of the time the battle is over before my enemy even knew that there was one.

So, yes, bloody idiots, the Nords. I guess those three Stormcloak soldiers or whatever it is they're called thought they could just kill me, steal my horse for their noble cause, and leave me to the ravens. Well, I guess they had a surprise... or not. I am not even sure their brains had time to register what happened. First day in Skyrim, and I'm already missing my homeland. But what was left for me to do there, after what happened in Morrowind? My family is safe for now, thanks to a friend, but if we ever want to return to our lives, reclaim our homeland and rebuild it, we will need more than prayers. Apparently Skyrim is a good place for adventuring, and given enough time I should be able to go back to Morrowind with money, supplies, and most of all, a small army of well-equipped and willing Dunmers to drive those bloody Argonians away. I just hope my family will be alright until I come back.