I would prefer longer levels to more levels. I've passed the period in my gaming life when I enjoy being a roving death machine.
Maybe a chance to move S.P.E.C.I.A.L. point and 5 skill points only if the cap got a boost. No more perks though, I had to plan what I wanted my character to be and there had to be perks that got left by the wayside that I intended to use.
I agree with all that prefer longer levels to more levels. Personally, I played FO3 and finished with a lot of the game unexplored, weapon schematics not found, etc but still at level 19.
Longer levels is a great idea because it can promote exploration and make the game more challenging (thus more rewarding). It would work better if the game had a rather difficult main quest (with slightly easier sidequests) because this would encourage the player to explore and complete side quests before continuing the main quest. Anything to discourage the player from running thru the main quest straight away ........ (personally I just avoid the main quest and go around exploring, but I think people would be more satisfied if more 'requirements' were in place for the main quests).
A perk to move a few SPECIAL points would be cool too. Same with the skill point thing - it would be interesting if you were able to re-allocate a few skill points at a time for every so much XP earned after level 30. So you'd have these pseudo-levels after 30 that enabled you to change your skills around (say to speech, for example) to something you didn't care about originally. Obviously it would be best if you could move just a few points for each level after 30, and not all at once. This would increase replay value and allow people to do things that they missed without having to start the game all over. It would also work with DLC pretty well too I think.
Personally with respect to levels skills and perks there are a lot of possibilities and room for change... not that I'm complaining at all, I love Fallout, but its cool to think what they could do with this game if they really want to improve it further.