This is a good idea, however. From experience of online tournaments (one of which had about 150 players in), I encountered several problems which I think will be relevant here:
Time Zones are a major issue, lining up people's free time is difficult, especially if you're giving them a short period of time to do it. I still had issues while giving people 48 hours to complete their matches.
Server crashes can also be a problem; a rematch will never be the same as the initial, since you knowtheir, pokemon, their movesets, their lead, their playstyle and can play against it. Also to a lesser extent, you have the "whoops I DCed" person. Though eventually I just said one DC and you're out (although it's easier to tell who DCed on PRO than DS).
Set-in-stone ban lists are usually a thing, but you need to consider if you need one? Yes, there's no legendaries, but we all know the current meta is very small on PRO, and it'll be the same pokemon in every team; maybe have a list of "You are limited to 2 pokemon from this list" type of thing. Either way, ensure the rules are fair, and make it competitive for everyone.