1) Generally, a little more in depth role play, and
2) specifically, the opportunity to develop a romantic interest with one or more NPCs.
I'm not talking about the one night stands and hokers of the previous Fallouts. Those interludes are fun RP diversions. But I'd like to see something a little more in depth, and more fun to play than the shotgun wedding in Modoc ending up with a totally useless spouse you kill or get divorced from ASAP.
What about taking the model of the follower quests from New Vegas and building on it, where the quest involves building a romance? Maybe the quest would culminate in a marriage, where major characters in the game could attend the wedding, something like the wedding in Rivet City? Maybe the PC could even decide what characters to invite, and doing so would help improve invited characters' attitudes toward you?
Just throwing out ideas here. I'm surprised and a little disappointed that Fallout seems comfortable depicting one night stands and acts of prostitution, but not a real romantic bond between characters. How on earth did they make New Vegas, with all its sappy love songs, without including a romantic subplot? I have to think the idea of a romance would appeal to a lot of the RP crowd, like me, for whom a big part of the game is the opportunity to live another life in another world. And those who find the idea unappealing don't have to explore that aspect of the game.
I'd love to find out if other people are interested in better RP and/or a romantic subplot. Can I really be the only one who thinks the idea is fun? Hopefully we can generate some discussion to show the developers there's interest. Maybe even give them some ideas.