There is a very simple benchmark to use for this.
You gain an ability point every 4 levels. You can wish for an ability point (max of 5 per ability).
You gain a feat every 3 levels. So why not allow people to wish for a feat? A feat is worth less than an ability point, since you get them more often. All you need to do is require that they have all the prerequisites and limit them to 5 feats total from wishes.
The argument of ultra powerful wizards once they can cast their own wish spells is nicely handled by the 5 inherent bonus cap. So just use the same concept.
You gain an ability point every 4 levels. You can wish for an ability point (max of 5 per ability).
You gain a feat every 3 levels. So why not allow people to wish for a feat? A feat is worth less than an ability point, since you get them more often. All you need to do is require that they have all the prerequisites and limit them to 5 feats total from wishes.
The argument of ultra powerful wizards once they can cast their own wish spells is nicely handled by the 5 inherent bonus cap. So just use the same concept.