It was only a matter of time, and now it's official. Starting with next week's downtime, Legacy Leagues will be coming to MTGO. Legacy Dailies have been struggling to fire on MTGO for some time, which is a similar problem that Pauper had as a format. Leagues revitalized Pauper, and I expect them to positively impact the amount of Legacy played on MTGO as well. It's difficult for everybody to set aside enough time to play a Daily- in particular when they fire at set times- and as such Leagues seem to be a huge success.
With the launch of Legacy Leagues you can expect to see an uptick in the value of Legacy staples on Magic Online. In my experience, Miracles has been the most popular Legacy deck online, and I'd definitely look there for some cards to invest in. Burn is the de facto budget option for Legacy, so I'd look there for pickups as well. I'm not sure how much play that Flame Rift sees in Legacy burn these days, but I do know that the card sells for about four tickets and sees a bump as is due to being from Nemesis- a set never actually released on Magic Online.
Dual lands are another place to look at with regard to investing in Legacy. They've basically never been worth much on MTGO, and Vintage Masters definitely drove the price down, but an uptick in play could cause them to see some significant growth.
Most importantly though, is that if you intend to actually play in Legacy Leagues, now is the time to buy your deck. Well, before the announcement of Vintage Masters was the time to buy your deck, but it's a bit late for that. If you want a Legacy staple on MTGO, I recommend buying in sooner than later.