Here are some good proposals that are at least possible but it is not clear how well will they protect us from bots:
1. Increase map size.
We've already established that this will cut out the current generation of bots, but will do little/nothing in the long term.
On long term sha512 hash function is breakable. Enlarging map in 2 4 finally ten times will make generation of easy maps is not possible. The complexity of finding an easy map shall be bigger than enumerating 2^128.
But keep telling that this is not viable. Yep...