Probably guilds should be balanced, i.e. if I push more the Thieves I lose trust by Priests...so one has to pick the guild to master at 100% or keep all of them at low level to have different rewrds.
Also the gems used in the shop should be used for something else than consumables IMO.
Easter eggs, or special skills that can be unlocked interacting with the environment in a certain way? For example pushing the runes in the obelisk at bridge level in the right order could give 3 extra stars, or a special skill for mages.