It seems easy to presume that if you grant levels or XP based on milestones then the game becomes about achieving those milestones. Maybe it's too easy and that's incorrect.
One of the DMs I play D&D with does fiat leveling when the PCs have done something substantial toward advancing one of their goals. Seems to work well at his tables. The obvious presumption those games are about the PCs pursuing their goals is consistent with my experience there.
Yeah, my group largely does milestone leveling or some variation on it when we play 5e as well. But I think it can be a very nebulous thing. Do players know what the milestones are? Or do they only discover them once complete and the GM then says “okay, you guys can level up now”?
Does that lack of knowledge hinder the players in any way?
I think this is where session zero steps in. Published or home brew I’m looking for queues as to what the games setting entails. Adventure paths highlight this for GMs and with players guides (big fan). If it’s sandbox, frame up the setting and take it from there.
I don’t need to be told “kill things”, “pick locks”, “talk to people” I’ll do that on my own. I just need a setting overview and off I go. YMMV
Sure, session zero and regular communication can help. Our group tends to know what the game is meant to be about. I’m currently playing in a 5e game of Temple of Elemental Evil. So, I know what the game is about and I’m engaging with the material. But I have no strong idea of when I’m going to gain a level. Certainly not with the kind of certainty that tracking XP would yield.
As for the XP triggers… it’s interesting. You seem to not want to be told to do the things you’d do anyway, things which you’d do in service to something else… something perhaps determined by you? But you seem okay with someone else deciding what your overall goal is.
It’s interesting. As I said, that’s how we level, too. And I’m comfortable playing the Temple of Elemental Evil game and I’m enjoying it. But I’m not really deciding what the game is, am I? It’s already determined.
But the games I play where the XP is more specific… the kind you think will lock things in to a specific cycle… are far more open for players.