No level cap, no levelling at all. Attribute and skills caps though.
There should be limited (realistic) attribute progression based on player actions. Ex: carrying heavy loads, wearing heavy armor, using heavy weapons increases strength; running, swimming, and jumping increases endurance; speechcraft, reading, alchemy, spell casting increases intelligence. (casting high majika cost spells increases willpower), etc [0 to 10 for attributes]
Skill progression should be near identical to OB. Ex: Using a sword increases your skill with blades, etc. Training should be perpetually available IMO even if its more expensive and involved. Trainers ranked as they were in OB. [0 to 100+ for skills]
Luck and perks should be set during character generation (birthsign, race, player option). I absolutely hated perks in FO3. How the hell do you become luckier over time? FONV made fun of that when you buy a cetain 'implant'.
It might also be interesting to have to 'maintain' skills and attributes or see them decline over time (slowly).
I agree wholeheartedly with this topic. And, I've heard true masters at crafts (i.e: Concert Violists and Guitar Legends) are still learning and getting better. There is no end to becoming better at something, and if dedication and time is given, then your skills should go past 100.
NPCs in gameworld should be treated the same way otherwise the PC becomes a god. Which happens in MW, OB, and FO3.