IMO... I hate the idea if used as "class" based. My rogue might well be a warrior built to kill, just by other means than muscle. Same for my cleric who might be focused on other things too. Class has moved beyond the olde straight jacket and lets not "help" the game with GM defining what you have to be.IMO it would help the DM develop the PCs in the way their players want them to progress. To become a better fighter you have to fight, to become a better rogue you have to steal and sneak, etc. I think it helps immersion by having players think more about what they character would do rather than having them do the easiest thing.
But, the same kind of "fix" to the threshold can be achieved with reducing the xp payouts of varmints of lower tiers. That allows the "plateau" to be reached where without tougher challenges the levelling stops (from world and pc.)
For milestone type leveling or session based leveling, this is easier by just not counting "lesser" sessions or goals.
I do not like and have not seen a lot of benefit from shackling advancement to "actions chosen." I have seen a host of problems when it was done and the GM decided the link, told the player how they had to run their character by basically defining a lot of things as not profitable for advancement.
Inspiration imo is the in game mechanic for gm saying "do these get cookie" not advancement.
If you the gm feel you dont have enough control already... So that you have to subset your player's characters choices to favor ones you approve of...
Do the work and put these nilestones in for class, sub-class, race, sub-race and backgrounds so that as a pc builds hos character and makes choices **they** are the ones making their choices on "what is it i do a lot of."
Might also consider adding traits like flaws, ideal, bonds to that too.