If we could review custom pieces, then it probably would be a lot different, as you would have to have your reasons on why a piece would be better off existing, rather than being "AMAZON+WILDEBEAST-RIDER+DUCK+IMMUNITY!!!!"
And even so, we probably won't at least in the near future, as our server developer is currently very busy working on variant features and fixing bugs.
The reason why piece suggestions do not work and usually get disregarded is that they don't add anything new or interesting from the current pieces or would likely not be used as much as the current pieces.
One of the most common examples is that of the camel+amazon or camel+queen. Sure, it's a piece, but how much does it change from the amazon? Nothing. It's also going to be less used than the amazon, because of it's strength and similarity to the amazon. Others include a "checkers" piece, which is obviously too weak, and sideways capturing pawns... which are weird
Pieces like the xiangqi horses and dragon bishop have their purposes and include much newer and exciting gameplay than their existing counterparts, which is why they were even considered. Forums like these have existed in the past, but nonetheless have died because of [pieces'] lackluster originality and have obviously been noted on why they won't be added