Knowledge upgrades would probably be your best bet

Currently that is the end of the line, no more skill points to be had.  But don't worry, by the long distant time when you reach that point plarium will have added higher levels.

My intial thoughts and current belief is, It would seem the situation is complicated  by the fact the question is somewhat brief, its answers though probably sound, are based on an interpretation of its actual want.

Does Heros skills refer only to those acquired by levelling up, which is probably most likely, or just any improvement to the hero's performance?

The first answer implies the second, the second the first.

As it wasnt stated, we do not know, what else the questioner has done in the game, or what  strategy is employed,so it's difficult to suggest specifics.

It's also simple enough to open up every link in the game and see what additional things can still be done, what they do, and  then asssign, if and where necessary, a preferred order for completion.