I've considered a RWBY game before, and my general thought is the best way to give the original characters breathing room is to set the game sometime before the events of the series, but well after team STRK graduated and moved on. Of course I'd like to hear Florence's thoughts on the matter.
First off, I'm really happy to see someone having interest in running this game!
To address the question at hand, I think what works best for me is to focus on an original story. I think an alternate timeline would be a good idea, that way we don't have to worry about contradicting canon or anything. It gives everyone involves more freedom too. I'm not opposed to setting the game before RWBY, though, if you think you can make that work better.
As for the when and where and how, I personally am fine with either, though its been a while since I've used Roll20. I'm okay going in whichever direction you and the rest of the group feels like going in. The forum would likely be easier, and more people here would likely be comfortable with it, though Roll20 would probably help keep things moving, provided we get an active group who are willing and able to show up on time.
Edit: Off-topic, but HNNNNG, this song:
God, Salem already seems like she'll be a great villain...