A bit of a noob to Runebound (let alone rpg board games) but I had a thought.
When Leveling what about to force an encounter level change (from green to yellow or yellow to blue etc...) every 3 or 4 levels?
And/or still allowing you to take a lower color level challenge from the level you are at however you get no xp. for it but do get the reward. Being an old D&D player I steal the idea from the idea that you can still fight lower level creatures but you just don't get the xp for it. I like this because it doesnt make the low level cards obsolete and also a way you can make a quick buck. To offset this (mainly for 2+ player games) you are "wasting" time when the other player(s) may be leveling up and moving towards the end.
Love the game, just thinking about more detail. It was rough coming off of Arkham Horror to this. Both are amazing games...now just to find a great Sci-fi board game with PCs (as opposed to a empire build set up).
Thanks to all on these boards, love reading them