Virtually every attempt at monetization on Roll20 is directed at GMs, not players. Players rarely subscribe to Roll20 because there is no incentive to. Pretty much everything subscriber upgrade, whether Plus or Pro, is focused on people running games and not people playing games. The sole exception is the removal of loading screen ads, but that doesn't seem to encourage many folks to pony up $4.99 a month. If we take the rough average of 1 GM per 4 players, you're only attempting to monetize 20% of your userbase.
Develop some subscriber features that make it worthwhile for players to subscribe to Roll20. Anything from special cosmetic token options to allowing a certain number of subscribed players to boost a game's features. Brainstorm it.
Right now, pay-to-play GMs are monetizing your player base better than Roll20 is. If nothing else, charge P2P GMs a monthly per player fee. Fifty cents or a dollar a slot for games that are marked as paid.
We want to see Roll20 run better, faster, and make some serious improvements. Focus on generating some revenue to do it by not ignoring the vast majority of your users.