I want to return to the game after a 2 month period of being bored. Never played as a Bosmer, Dunmer, Khajiit or Argonian (not Redguard either but don't want to play as a human). Could anyone suggest a cool build? I haven't done sneak before either, as I have never enjoyed it before, but I am up to giving it another go.

I haven't played as Redguard yet, but personally feel bored with the human races now. Bosmer and Dunmer are my favourite looking two merely because of the whole elven situation surrounding Skyrim. But, I can't think of a build. I don't want to do a generic thief, mage or warrior guy. I like "hybrid" classes which utilise a variety of skills. They tend to feel more...unique.

Is nobody here able to suggest a reasonably unique build, which hopefully utilises one of those races? I like the idea of a cool sneak class?

My main is an unarmoured healer with a sideline in undead hunting. Just a twist on Spellsword, but mastering Restoration above all other magic schools does provide a different way to play. She's Altmer but it would suit a Dunmer from the Temple. Main skills are Alchemy, Restoration, Enchanting and Speechcraft. You need some Destruction (fire spells and necromage perk v. undead) and One Handed just to survive and cause damage, but much less perks than the main skills. It's a challenging build to play, the Restoration and Alchemy don't help when you get one shotted, but it teaches you to be careful.
Role play wise, no poisons, no soul trapping (only found or purchased), big use of fortify health (the Mage's way to stay alive), always give to beggars, turn down any criminal or 'evil' Daedric quests.

I kind of want to play as a sneak-type character...

Play on Master Mode - This isn't a major thing.. But you will feel much more accomplished and its quite fun.. Stay clear of Dragons and Dragon Priests until you are very strong.. Like 120 dmg per hit..

Be a Khajiit

Put your skills into Sneak and Archery

Also put it into Alchemy and Enchanting to make you better

Its fun, I usually play this build.. Also use a crossbow, its quite strong I realized..

I know the feeling, I haven't played Skyrim for months, instead, I've been playing oblivion.

Anyway, you may be interested in this http://theskyrimblog.ning.com/group/character-building/forum/topics/character-build-the-stalker

Argonian poisoner. Sneak, archery, daggers only, light armour, pickpocket just for 'that' perk and poison side of alchemy maxed to the hilt. Sneak and archery even without the poisons can be a bit on the easy side, so play Master or Expert. Make it your mission to dispose of Dunmer for past crimes.

Or this : http://theskyrimblog.ning.com/group/character-building/forum/topics/character-build-the-burglar

I kind of want to play as a Dunmer. From a role-playing perspective, how can I do both the Dark Brotherhood and Thieves' Guild quests as these two factions have opposing views on violence?

You could make your character not care about the two factions' morals; rather, he could just know the "evil" factions make him the most money, and he'll do whatever he can to make more money.

I always view joining another guild in addition to the DB as a cover story. For example, if you played similar to the old 'Agent' class (in Skyrim terms Illusion, Archery, Sneak, Lockpicking and Speechcraft), your cover would be a member of the College just trying to master illusion spells. No one needs to know what your night time job is