The objective of the Shard Shop, is to top off those characters we may be needing a little extra push. I'm not going to complain over the fact we only get 6 choices per 21hours, with the possibility of only getting 2 shards per slot.
I'm going to complain that, given the fact we only get this limited amount of choice, that characters we already have at 7-star do not become available there. If the objective of the Shard Shop is to trade our extra 7-stared characters shards, then it makes no sense whatsoever to have them on the Shard Shop. Can we please remove them from there? Only present characters we haven't 7-stared yet.
Bonus: the characters that appear on the shard shop should be based on our current roster. If we have a 6-star with 15 shards left to get him at 7-star, then those characters should have a higher likelihood of appearing on the shop, versus characters we haven't yet unlocked. This would make the limited selection of characters much more appealing, and the shop would make a lot more sense that way.