Speaking of unpleasant and generic underwear, that reminds me of Mabinogi Heroes (another KRPG), which allowed players to customize the underwear of their characters (which was an important Cash Shop item for the F2P game), and if they chose not to, no clothing/armor they would equip would "hide" the generic/cotton underwear, so if you were going for style (and this being a KRPG, you WOULD!) then custom undies it was! Naturally, stylish isn't the goal in mind for surviving the post apocalypse, but if a similar system were implemented in the next Fallout game, I would not have any qualms with that. If anything, it leads into one thing I wanted to see in the last 2 games, which I only just remembered; stat PENALTIES.
If you're running around buck naked, some of your stats should probably take a hit. Namely, EN and CH. While I don't think that putting on a stereotypically 50s-ish outfit should give +1 AG and +1 EN, I do think that in certain weather, not wearing clothes would be a detriment to your immunities, and putting on a really snazzy suit COULD improve your CH overall. So, if you had a very nice looking suit on, or some lingerie, but the ugly underwear was getting i the way, then no CH bonus for you (or make it smaller), and likewise if you're one of those players who thinks it's "fun" to mash the space bar and jump every 2 feet while wearing nothing, you deserve to be slapped with a stat penalty. Provide a suitable reason to wear armor at ALL times, as well as provide suitable reasons for every action a player undertakes, and it all helps the player forget that they're playing a game. The more convincing, the better!
Another thought I had, which was related to my desire for bonuses and penalties depending on what the player looked like, was for a tad more character appearance based on stats. I was reminded of Fable for both of these things. Not that I want a Fallout game to become a one-town-management simulator, like most Fable titles seem to take on past a certain point, but if in addition to losing some stats people would mock you and few would want to talk to you while you were naked, I can't imagine that would be a bad thing. Likewise, if stats like ST, EN, and AG had an affect on your character's body frame, that would be pretty nifty (and present more variety compared to the one-size-fits-all generic character frame). Perhaps even go one step further, and limit the degree of facial options based on your CH. The game I played which I felt handled character creation visuals THE BEST was Dragon's Dogma. It didn't give you sliders to move back and forth that resulted in a very unnatural-looking creature for a character; it provided dozens of body parts to pick and choose from. Mix and match at your leisure. The result was that you could really come up with a surprising variety of appearances, yet none of them would look disturbing in the Uncanny Valley kind of sense. If the next Fallout game used that system for determining facial appearances, then I don't see why the AMOUNT of options (particularly, the "prettier" ones) couldn't feasibly be limited by how high my CH was. If my character's lankiness, height, and obesity were predetermined by the stats I chose, that would really lend itself to a remarkable tool for immersion's sake.
Then again, all of that comes from ideas I amassed from OTHER games, not ideas from the last ones that I'd like to keep...