CapnZapp
Legend
Sure, but now you've switched discussions to NPCs.Bad, bad idea to have proff. bonuses increase.
If you want higher numbers - epic abilities allow raising stats to 24 for an extra +2, which is a high addition to the game limited to +11 for most and +13 for Barbarians (before the addition of magical items...).
On the multiclassing issue - doing that doesn't sit well with the core aim of Bonded Accuracy.
However, I have introduced (in order to keep it nominally equivalent to getting epic boons instead) a system for gaining either the core abilities of another class (one per level in order) or just those of a subclass of the same class the character is as an epic 'pseudo-multiclass' option.
It works well. I created Mordenkainen with that and a mix of epic abilities and subclass goodies and he ended up an Epic level Evoker/Abjurer Wizard with an extra 9th level spell and 24 Intelligence - perfect!
He certainly felt appropriately powerful with +13 before magical items on his spell DC base and spell attack...
Since level 20 character are already supremely awesome, and since they can already break all kinds of limitations of the game, I would say it's safe to assume the normal prudency (beware high profiency bonuses, mind the bounded accuracy etc) has already gone out the window, so go wild, knock yourself out!