Currently, in online chess, staff can create tournaments with unlimited members and platinum and diamond members can create a tournament with maximum 100 players. It would be nice if it would be increased to a higher number, say 200.

Agreed.

Starting on a specific date w/ unlimited members would be nice

Try organizing one with 100 players and see how long it takes to meet the quota before you worry about being able to set up bigger tnmts. Check out the 3 tnmts I'm currently organizing 2 for 25 players and one for 64 - and I've had prior experience in waiting for tnmts with 100 players to meet the quota.

From now on I'm limiting my tnmts to about 25 or fewer players, esp since I'm mainly interested in class and/or thematic tnmts, both of which automatically "exclude" a lot of otherwise interested players.

+ 1

There are options to start and cancel tnmts you've created so I'm guessing you can start a tnmt whenever you want to - regardless of how many players have entered. If this is correct then you could announce the tnmt starts on xx/xx/xx when you create it.

Yes, but it has a limit of 100 players