I like the idea of having some of the games have a longer time to complete. I know I've had a few games that weren't finished in time, and some more that almost didn't finish in time, so that could be beneficial.
I don't see why it's a problem if top players want to get the best scores.
To me, it seems like Open Class makes the least sense to get rid of, since it's the most common (or was when I last checked), and probably the easiest to set up?
My guess is Conquest class makes less of a difference as to whether new players start playing GOTM than the difficulty level of the current GOTM. It might make them more likely to try a higher level.
However, I am at most an occasional GOTM player, so don't weigh this too heavily.