The criteria on what group belongs where is up to the TO, much like how stage lists for smash in general is up to the TO. It's just as complicated on trying to figure out whether a stage is starter/cp worthy or banned/cp worthy when said stage is pretty much borderline, ya know?
on top of that group criteria should be relatively simple to be more encompassing and cultivate stage diversity within the groups to help further prevent ban invalidation.
2-3 stage bans just complicates things more than it should, especially when there aren't enough bans to cover enough of stages to prevent the problem of ban invalidation.
With group banning, you're essentially banning by traits determined by the region/TO.
It's also far more simple than multiple bans, at least i think so.
First: thanks for being mature on the internet. :D
Second: the stagelist being arbitrary TO decisions has always been the case. The problem would come from when tournament A groups together A-E, F-J, and K-O while tournament B groups ACDEF-GHKMN-BIJLO and tournament C groups AEFJO-BCGHN-the rest and tournament D groups ABC-EFG-HIK-LNO-DJM and oh no I've gone cross-eyed. FC had a nice solution--keeping them in rows on the stage select screen--but can every TO do that? Moreover, can you expect the community that apparently can't learn how Randall works to keep track of all of it?
I mean, I'm all for regional ruleset differences (a truly good smasher should be able to win in any ruleset), but I could see problems.
Third: I wouldn't be worried about ban invalidation nearly as much as I'd be worried about CP invalidation. Remember that I come from a Brawl background (you probably know this, but in case anyone here didn't realize that by now); in that game, most places run 10-13 stages AFAIK, with one ban, and other than the thing with MK on Brinstar and RC (which I'm going to use in an example in a minute here) nobody really had a problem with it. Counterpicking is a part of the game; why include it if you're going to neuter it by banning half the stagelist (either by banning the stages outright or providing so many stage bans that they don't matter)?
Upon thinking about it, I'd like to propose a compromise:
1. Wait for the metagame to develop for a while, or maybe for the full release, IDK.
2. Keep track of what stages are used for what.
3. If a few stages cause the same problem (eg. MK being very good on both RC and Brinstar in vbrawl) group them.
4. Allow the option of banning a group together, or any individual stage.
So if (semi-arbitrary example) Metal Cavern, Randall, and GHZ all turn out to make Ike really OP together, you could group them and give the option of banning the group instead of banning any one stage.
You should not, however, attempt to group everything, because not everything needs grouped.
In other words, at tournament L, your groups are:
ABC-D-E-F-G-etc. and you ban two.
Sound good?