In our group, while playing D&D 5e we use the system in which every character that made to next story milestone gets a level. It is good because the party is at the same power level and it is easier to adjust level of challenges. Also, gaining a level is pretty big thing and denying it to a player because they show less commitment than others (say, player gets less XP for role-play and their character doesn't make it to next level) wouldn't be comfortable to me.
But "equal for all" advancement system has its downsides too. After 2 years of playing I feel that the party has little incensive to show more commitment. I'd like to see more role-playing, more devising clever tactics and looking for non-obvious solutions.
What systems could you propose to provide a player with lesser-than-level reward for role-playing and other things I mentioned? I'd like it would reward only the player who deserves it so that every player would be motivated to do one's best.