We actually discussed this a lot at one point, but we backed away from it for a couple reasons:
1) It could end up being a little disingenuous. Basically, we want to work on Roll20. We've got people who are paying to keep that happening. Do we hold them hostage with features until we meet certain metrics? Or do we develop at the pace we're able?
2) We don't want to be tied to a development cycle that relies on just accomplishing sexy features again and again and again. Right now, there's a half dozen whiz-bang things we know we're doing in the next few months. After that, there's nothing that's even crossed our mind as a new feature... and we're REALLY okay with that. We're trying to maintain a simple interaction tool. So we want to take time to keep it clean, to debug it, to better polish and calibrate it. That doesn't necessarily sell well.
That said... the supporter bar has helped us establish the basic mission statement of pushing for subscriptions, and we're really pleased with the results so far. The gradual climb we're seeing is promising. That doesn't mean we're completely "out of the woods" or safe to stop innovating, so we'll keep on trying to figure out ways to make supporting us better and easier, and you keep spreading the word!