[Suicide Junkie]

"Leaky XP" would be good if it involved, say, ships and fleet experience leaking away over time.
Having to brush up your crew's skills after a year or two on a peaceful patrol would make sense.

Having bonuses spawn from something you did in a previous game (racial experience) has already been shot down as a bad idea in SE4.
I seriously doubt that anything SE5 is doing will change that.