Bring back old concepts and add new ones for Guilds.
Two things I liked about Morrowind's guilds; Level requirements and Guild conflicts.
Level requirements make sure that you are proficient in certain skills if you were to advance in a Guild. For example, you need to be skillful in magical areas if you want to move up in a mage's guild.
Guild conflicts were awesome, because it gave the game replay value. One could not join every guild in one play through because certain guilds were enemies and it wouldn't make sense to be in both(eg. The Great Houses). It wouldn't make sense if someone was part of House Telvanni and then get a quest from the Mage's guild to kill all the Telvanni councilors.

A new concept; Gaining responsibilities as you progress in your faction.

As you progress in the faction, you should gain responsibilities and benefits. Perhaps in the fighters guild, at a rank near the top like a captain of some sorts, you can gain control of the fighter's guild in a certain area. Control of that area of the faction would come with regular challenges from other fighter's guild members, who want to take control of that area, so they challenge you to a duel. That being said, if someone doesn't want to be pestered with the constant challenges they receive, they don't have to control they area at the required rank, but can choose to. With control of the portion of the guild, you would receive a steady flow of income and would be able to have members in your area become companions.

I think guild quest lines should be a LOT longer than they are. Being done with the fighters guild quest in OB in less than a day is just downright ridiculous.

Yeah, ALL the quests should be alot more involving. Or at least a string of quests. E.g. that rat exterminator quest for the first fighters guild quest was done well, and it wasnt just go in, kill all the rats, and leave. it had a twist and wasnt too short. However all the quests after that were pathetic "go for" quests or just extermination quests.

"Rats! In my basemant! And somethings killing them! Ohhh my poor babies".

Longer, maybe, but better than Morrowind's guild quests. Not just walk to a certain location and get a certain item.

I might be rehashing old ideas but...
Mix Morrowind and Oblivion!
Oblivion has good graphics and a much better combat system than Morrowind. However, Oblivion still seems like a step backwards to me.

1. The Oblivion main quest was far more bland than Morrowinds (not saying Morrowind's was great, but it was at least original).

2. Also, doors that require keys? Someone tell me why, unless it's magically locked, I can't find some way to get past a wooden door!
Also, unkillable NPC's? I'm sorry Bethesda, but that is the kind of rushed/lazy programming that reminds of an EA product.
Go back to the open ended style of Morrowind. Many of the quests in Morrowind had back ways to complete them, should you choose to kill an important person. Really hard to role play an assassin when all of the high profile targets can only be knocked unconscious.

3. Bring back role playing to TESV. Certain aspects of Oblivion just took you out of the suspension of belief. Such as beast races with normal feet just so they can wear boots like the other races. And those little useless interesting items laying around Morrowind that made the game interesting... (Belladonna berries, anyone?)

4. Also, the skill system in TES IV is pathetic. No unarmored (everyone in Tamriel forgot how to dodge apparently), anyone who has ANY training in weaponry knows short and long blade do not have the same mechanics, and anyone who has eye sight knows an axe isn't a blunt weapon. :rolleyes:

5. All over, TES IV had a bland feel, most of the armor was uninteresting, the landscape and ruins were largely the same. Sure half of Morrowind was gray and dead, but at least you could tell the difference between the West Gash, and Bittercoast regions.

I would love to see TES V bring back the elements that made Morrowind groundbreaking, without losing the elements that made Oblivion good.

Post limit - I'll put the new one up in a couple of minutes.