This is a very simple suggestion.

1. All campaign maps should give EXP, and maybe a max of 5 items and runes.
2. All ethereal maps should give NO EXP, but give lots of items and runes.

Therefore, we play ethereal maps to get more items and stuff, and campaign maps to levelup.

Think about it. It fixes the issues with balance, and cuts down dramatically on the grind. Endless mode, once again, becomes the primary way to obtain EXP.