One of the memorable things about earlier versions of D&D is that every level has an associated title — first-level wizards are Prestidigitators, fifth-level fighters are Swashbucklers, third-level assassins are Waghalters (not really all that intimidating). But with the exception of “name level”, when you top out the title system and (in 1e, at least) can build a stronghold and start accumulating followers, none of the titles mean anything: just because you’re a Swashbuckler doesn’t mean you’re actually swashbuckly in any way.
So why not fix that? Earlier editions don’t have much of a skill system, so you generally end up using ability or skill checks if someone wants to do something. Well, take that, and then if the character’s doing something related to their title, give them a +4 to the roll: fifth-level Swashbucklers get a bonus if they’re trying to make the ladies swoon or disarm a troop of guardsmen, but once they hit sixth level and become a Myrmidon, then they get bonuses for bodyguarding and military formations.
Note that this plays well with the old-school tradition of characters dying frequently and starting off again at first level: now there’s actually a reason for you to want a second-level magic user around even though the rest of the party is fifth level, because the second-level guy is an expert Evoker and that might be just what you need. This also helps compensate some for weak fighters, another common complaint: they’ve got titles like Hero and Champion that can be applied to a lot of different situations.
This is pretty simple but I imagine there are a lot of other interesting things you could do with level titles. I haven’t even mentioned cleric titles, for instance, which clearly imply steady advancement up a religious hierarchy and the wealth and power and politics that leads to..