One suggestion that I would like to see (as mentioned in chat) would be party-based abilities. Whole party buffs like speed or damage increases and the like.
I'm not sure what would be possible while keeping everything as simple as possible (ie, no class-based specialties)
you do know this is a idle-webgame ?
This feature is somehow already implemented, as upgrades like "HP Reduction" and "Time Extend" against mobs, plus some others TreasureSpawns-related are passed on to the other member of the party (the upgrades with the higher level among the two players are taken in consideration). Plus there's an Ascension upgrade that raises XP and gold earned in battle while in party.
I'm mostly meaning some other variable to compare potential party-mates. Right now I go for the highest level on the assumption that they'll add more DPS
This is definitely a possibility on the long-run, especially with the new addition of challenges and how they'll require strategy in order to complete the extremely difficult ones in Duo / Group.