There's a big difference between what is the bare minimum I could play with and what is the minimum that would tempt me to leave my current VTT (Foundry).
For bare minimum, I guess what Owlbear Rodeo offers: dice roller, map with tokens, handouts. To be fair, it offers character sheets for many systems and simple fog, but I can live without those and have done so.
I run Vaesen, Shadowdark, Dragonbane, Call of Cthulhu, and Delta Green. What I have with Foundry that I would not be willing to give up: character sheets with integrated rolling of skills and attacks, initiative system automated, map with easy to use dynamic fog of war, handout system, background music, special effects for splash screens, graphic dice roller that is sync'd between players (ie, everyone sees the same roll), pointers, measuring tools, drag and drop statted creatures, popout character sheets (ie, separate browser window), and support for the games I play and enough popularity to be assured that most new games will be supported.
What I don't have right now is video chat and full hosting. I could get those by switching to The Forge from self-hosting for $10 a month and I contemplate this. The feature I miss from roll20 is some kind of integrated looking for group feature.
I think it is very very difficult to enter the marketplace now for a VTT. Roll20 and Foundry had a benefit of a head start. But this is a mature market. I don't think a newcomer can get away with slowly building out features other than as a sort of fanboy cult.