Like I go saying since several months giving some "boosters" to who stays single or little classed is viable.
And to make you earn it is easy: you'll get a window like the training points one but with "prestige skills" (those that empower you till you are like a multiclasser even if you are not). When you reach say 10M unspent exp over level 100 the first row of skills "lights up" and you can press one of them (they are like buttons) and pronto you are now with 0 exp over level 100 but got your shiny "extrabash of uberdom" skill.
Next time you get again 10M exp over 100 the remaining button in that first row will light up again. If you don't use them, after i.e. more 20M exp only the second row button corresponding to the skill you chose earlier will light up (the others won't since you did not advance in the first row of them) and you'll be able to press it and gain the enhanced extrabash of uberdom. And again, 20M exp are subtracted, "eaten" by the act of getting the skill. And so on.
Doesn't this feel right? Maybe after5 months I repeat myself someone could implement it...