Maxing out buildings and unlocking gears and classes are the only valuable unlocks, ressources are over abundante. You don't even need to grind higher difficulty than Noob. I never had to use the Market conversion.

For game longevity and replayability, it would be nice to tied progression to Mercs/gear development or high end achivements. At least for the last stages of buildings developments.

Somethink like
- Infirmary Lvl 10 : Finish all raids with x Mercs at Legend difficulty.

We rapidly have nothing significant to go for except the brain numbing achievement "Successfully clear on every class, every map".