So let’s say you earn some XP.
In a classless system you spend the XP directly on skill points, ability modifiers, etc. If you want a character attribute improved you use the currency of XP points.
In a classed system, you accrue XP and when you have enough, you gain a new level with new cooler stuff.
What about a magic item creation system that creates a type of hybrid between these two ideas. What if there were classes but upon receiving XP it was not automatically applied to your level? What if you received XP, but then kept it, in a proverbial “bank”?
From the ‘bank’ you could certainly “cash in” your XP to put toward your next level. But with it you could do other things as well.
Because I’ve been contemplating it recently, what comes to mind first is Magic Item creation. What if you could take that ‘banked’ XP and apply it to an item. Once the item had accrued enough XP, it would qualify for an “item enhancement feat”. This ‘feat’ would grant the item magical properties.
The next thing that comes to mind is powering High Magic spells. Much like in D&D, some spells have a XP cost. This cost would come out of the ‘bank’ rather than the XP spent on the character. This way, casters could not cast themselves down a level. (Not that I know anyone that this has actually happened to, but still theoretically.)
Now to address the comment made on the last magic item post, The reason I would have them be feats and not an "imbue" ability is, that if I as a player don't want to imbue something with magic power, i don't have to. With a feat, I can decide, "oh I don't want to magic up my sword anymore, I think I'll take 'Mounted Combat' instead".