Recently, I've been playing a lot of Fallout 4 and it made me think about which concepts could be adapted to TES. While the series are very different, there is enough similarities that I think the question is relevant. Some ideas that that come to mind:
Disposition of Companions
Although I'm not sure it's necessary for a box to pop up in the upper left hand corner every time a follower approves or disapproves of your actions, I love the idea that companions are unique characters with likes and dislikes. It's absurd that every follower will tag along blindly regardless of your actions. Most of all, follower disposition gives a reasonable way to regulate the player's behavior. Fallout 3's Omnipotent karma mechanic went too far, stealing anything, even from a bad person without witnesses, pushed the player towards the dark side. Skyrim was the opposite, you could steal from the nice, old alchemist every few days and she'd be just fine. I would love a system where stealing repeatedly from a store could put them out of business, but until that day, a judgmental companion is the best way to make me think twice about robbing every NPC blind (or encourage me to do so if I have a follower who likes that sort of thing).
Fallout 4 also made an excellent decision by giving some companions an unique quest that became available when their disposition was favorable enough. One of the best aspects in both series is those moments when something seems simple but leads to a much more complex quest.
Meaningful Faction Membership
Fallout obviously has the advantage here as the games have a defined ending and force the player to choose sides between the factions. This mechanic would go against the TES' ideal of allowing a single character to do pretty much everything. I wouldn't like to see this implemented in the next TES; however, I would like to see my faction memberships have an impact beyond the guild hall. Guilds shouldn't be merely places to get some quests and have access to merchants and crafting stations. Belonging to a guild should be an essential part of my character's identity. If I wear the Thieve's Guild armor, people should react to that. Honest people might shun me, but shady NPC's might be more willing to invite me to join in their schemes.
The one concept I'm not sure about:
Settlements
Settlements work perfectly in a game set in the aftermath of global annihilation. Tamriel is more developed and there are less logical opportunities to build a small village. Certainly, if such a mechanic was included, the number of possible locations would have to be limited. Maybe this could be possible in a few smaller holds similar to the Hearthfire homesteads. Or (I'll need the help of lore experts), maybe there could an untamed region in whichever province the next game is set in that would necessitate the building of forts to expand the ruling party's influence. These examples aside, I'm not sure if the idea could be seamless integrated.
Anyway, I hope to hear ideas from people much more knowledgeable than myself in both series.