My suggestion is simple, although I suppose implementing it might not be as much: reverse the generation xp bonus, so, if you are gen. 1, you will level up the fastest, and, as you retire and your generation increases, you will level up slower.
This way beginners will catch up with veterans much faster, and, with the new standard double xp, levelling up with a high gen wouldn't be so bad anyway. It will also make levelling up alts less of a pain.
I've just realised that this could bring problems, like people abandoning their high gen. characters to retire with new characters, that would level up faster and there would probably be an heirloom boom.
How about, instead, the generation xp bonus is removed and you get max xp since gen 1, so the grind will be less painful for our alts and for beginners?
Currently, levelling up a character with a low generation is a pain, compared to one with a high generation, and maybe this made sense in cRPG origins, but, today, apart from the level, gear and skill difference between veterans and new players, the latter also have to play much more to get to the same level as veterans.
I think you should level up as fast as someone with max. gen. since the beginning, and leave the generation stat as something aesthetic (to count your retirements).