Challenge: Hold > 400 resource cards in your resource inventory
For seasoned players looking for a bit of a bit of a tougher challenge, the challenge is to obtain more than 200 resource cards over your resource capacity (staying within intended gameplay, e.g. no refunding of purchased capacity). This may be only possible for certain seasons such as the current one (S17 / S18). Here's a screenshot showing 394 resources, or +194 over capacity, but I'm sure someone can beat this!
A related challenge would be the theoretical max building card inventory size. Unfortunately, currently the game imposes a maximum of crafting 30 building cards, but if this limit didn't exist it would be possible to craft 76 monuments from the above resources and go +75 over the building card limit. +77 should be possible with some luck and maybe someday +80?