I use Fantasy Grounds and Foundry and have previously used roll20, but I don't have a favorite. They all do some things well and others not so well, so IMO none is ideal.
I'll ditto being able to support other game types like tabletop miniatures and board games. With true Player vs Player support. I've done modding to get miniature games working on VTTs, but with them being so Player Vs GM based, it's always clumsy. There's Tabletop Simulator, but after a decade it remains a poor system for hosting games - wish it had improved/evolved because it can sure look pretty, but it just hasn't.
I'll also ditto better mapping tools. None of the mapping tools in any VTT I've used (ran Maptools for 2 years too) could entice me away from Campaign Cartographer or Dungeondraft. I can create > export > import > setup a map with either of those apps, far faster than I can monkeying around with their native mapping tools. Heck, even the simplest of maps are usually a pain. And it's not that I haven't given those VTT tools a concerted college try - I have.
Alternate, non character perspective Line of Sight. This goes with my desire to have a VTT that also supports miniatures and boardgames. Current LOS in VTTs is based around a character token persepctives and because mapping apps like Dungeondraft now export to the VTT those file formats, I fear it'll get harder to change. I'd like to see VTT LOS tools that don't just obscure/block, but support how LOS is handled in other game types.
Better viewing perspective control. We've had support for all sorts of player-controlled viewing and map scrolling for decades in Real-time and Turn-based strategy games, and I'd love to see it in more VTTs. It sort of works in TTS, but so much else doesn't in it. I realize it's budgeted for in RTS and TBS development and not for VTTs, but with so much code libraries and talent out there for how to do it, it puzzles me that not more is being done with it. And no, the 3D camera in FG doesn't come at all come close to what I'm thinking.
Apps on mobile platforms that allow for purchased game content to be viewed on mobile devices. And no, roll20s ability to display a PDF purchased from drivethru dosen't come close to addressing this. And if such viewer apps are too pricey to build, how about at least universal 1st and 3rd party license sync'ing. I'm thinking like Paizo does with some VTT stores, so you at least get a free copy of the PDF. I'm not bothered by the effort of having to go the the publishers website and dowload the PDF. Bits & Mortar has been around for years for supplying free PDFs for in-print book purchases and it's simple to use. Why can't we have the equivalent for VTTs?