I think maybe it would be nice to have special challenges where you’re given X units at levels Y and Z gold, and you do whatever you want with items and skills, or perhaps just a bunch of gold and spendable experience points, with “level up” buttons and such.
The trouble with the grind-at-will approach is that by the time you do all the previous bonus levels you are ready to handle, the next standard map forward might be quite easy, and you can float along on an air hockey table of drifting difficulties before hitting an obstacle and bouncing back. A map where you have a definite, fixed amount of resources and some given assessment of the challenge involved would get things under control more. An “extreme” map from early on isn’t really extreme if you’ve almost beaten the main game, and a “hard” map from near the end is much, much easier. It isn’t really a bad system overall, but it makes it impossible to test yourself against something the developers truly consider balanced for a set difficulty.
Maybe the rewards should be appealing-but-not-exclusive, I don’t know. Since the philosophy seems to be not to leave mediocre tacticians totally behind.
Thoughts?