The problem with passive skill gain is that the player would start acting really bizarrely in battle if they want to have any control over their stat progression (which you really want, considering how the battles work). Need more fatigue? Better hit-stun an enemy with some guys so that you can run around for a bit. Levelling hitpoints (being near death?), resolve (passing morale checks maybe, but that makes it harder to level if you have less resolve) and initiative (don’t see how) would be even more difficult.
The traits gained through achievements (or failures) of a merc is something I’d also like to see. Would have to be minor bonuses though, since you don’t want to encourage specialisation too much.
Game isn’t coming out of Early Access until next year anyway, so no rush to upgrade. I don’t think you’ll need a beastly machine for it either, as this seems to be the kind of game pretty suited to laptop gaming. :)
Agreeing on that must have potential. In terms of fun I’ve had, it’s already been worth the price of purchase and if they further build on the basics it can become real classic.
EDIT: Seems I got sniped. :P
