Uxie would have been pretty good in this format, imo. On one hand it would be cool because it would increase consistency in a format that really needs it, and balance would exist since every Uxie you play is potentially a free prize with Catcher in the format. I feel it would have been great in BLW - BCR, but now with Hypnotoxic Laser + Virbank I think Uxie would make first turn damage too high too often and make going second even more crippling.
Claydol would be nice, or something similar to Claydol. 70 HP Baltoy would be fantastic. I can't help but feel setting up a Stage 1 that will probably only survive for 1 turn isn't rewarding enough but maybe in this format thicker Claydol lines would thrive (ie 4-3 Claydol) since Eels can pull it off.
I would really like to see Vileplume UD or some bench sitter Item Lock return. It sank in for me that we'll probably have Lasers + Virbank in the format for the next two years... and they were released into a format where they can be ran virtually unopposed. Item lock would completely halt them, and Catcher, and allow other bench sitters to survive. In a super fast format that's so reliant on items, racing to set up an Item Lock seems to fit perfectly.
Something like Hariyama EX would also be cool for the Stadium lock. It's definitely a no from me on BTS and old Rare Candy. If BTS were ever needed, it could be included as an effect from an Ability specific to that Pokemon (similar to Reaper Cloth Duskull).