Doesn't it get boring having 100 skill in EVERYTHING that you use, or having done everything?
There is no set definition for "everything" in an Elder Scrolls game.
There is always something that you can accomplish. Collect every book in the game, collect every piece of clothing, armor, weapon/item.
Clear every fort, ruin and/or cave...
As for me, I find having multiple characters to be counterproductive.
I put my heart and soul into one character, and make it a goal to conquer everything that can be completed. Every quest, every side quest, clear out every dungeoun, fort/ruin/cave.
Collect every item, or 2 of each, own every house, own every kind of horse.
The only reason that I would have another character is so that I could join the other side in the war, marry people that I couldn't marry before, and otherwise do the things impossible for one character to do in a single playthrough.
This business of having multiple characters is just a sloppy approach in my opinion, like annoying clutter.