This is probably already too late to matter, but I was just looking over the current capstones and thinking: maybe (in an ideal world), "capstones" should apply to any pure-class character, whether level 2 or level 20?
Obviously this wouldn't work with most of the capstones as written, but some of them already seem the kind of low-level buff that would be reasonable at any level, and others could be reworked or scaled to function that way.
Aside from adding flavor to pure-class characters before they hit cap, I also think this could be better balance-wise. After all, your splash in Fighter/Rogue/Monk/whatever adds benefit immediately - why should the pure-class characters have to wait until 20 before they see an added benefit from their choice?