I don't particularly care how the mechanics work, to be honest. I just want to be able to load up any mod, import my level 15 party, and have a challenging game which I come out of at level 25 (for example). Then I want to import my level 25 party into another mod and have a challenging experience which ends with my characters being, say, level 31. I want to infinitely use the same party forever, and keep levelling up like I do in World of Warcraft where I've spent years getting to level 90.
I've been working on a Grimrock 1 mod for years in my spare time which has 24 levels and is suitable for characters imported from the end of the main campaign - I've done it by creating areas with lots of monster fights and battles interspersed with hundreds of puzzles. When you solve the puzzles, or go to the next dungeon level, the game gives you experience points as rewards as happens in other RPGs. In this way, you level up as you would in the main game. Yes, it's artificial but it works
