In the previous TES games I've played - Morrowind and Oblivion, both allowed the player to join several factions (guilds sometimes) and rise to the head of each. This proved to deepen the game experience well beyond the main quest line...and made the game a lot of fun. I also like being able to join each one independently of the others.

In the future TES V game, I'd like to see the following factions included in the game:
-Fighters
-Mages
-Thieves
-Black brotherhood
-Arena
-Knights (privately hired to be a rival to the Dark Brotherhood)
-Necromancers (the rival to the Mages)
-Merchants (making use of the ships...like those seen in Oblivion docked at Anvil, Imperial City, etc.)
-Pirates (rival to the Merchants...also with ships of their own)
-Imperial Council (some form of political position where the player rises up the be elected or appointed a noble who governs a city)
-Guards (player starts out a squire and rises up to become Captain of a city watch or even higher)

These are just some general ideas. I was really excited at the possibility of expanding the game to include more factions and actually having more of the factions be rivals to each other, the way the Mages were fighting the Necromancers in Oblivion.

If anyone feels they want to contribute more ideas for factions and ways to make rivalries, feel free to post. I look forward to discussing this with members of this forum...and MAYBE the folks from Bethesda will like to read about these ideas also, assuming there's still time to include anything from here in TES V...

Actually we have an http://www.gamesas.com/index.php?/topic/1120296-tes-v-ideas-and-suggestions-183/ for TES V ideas, you're welcome to repost these in there