To answer the thread title directly, for me a new virtual tabletop would need:
-3d, rollable dice, with stranger types like d30s, fudge dice, and others that you can choose from to fill a "dice bag" for a given campaign. The ability to customize the exact look of the dice in your dice bag by making them look like metal, stone, fire, or whatever. Optional special effects that you can see but won't annoy others.
*The ability to roll in the open or in private.
*Customizable character sheets that fill in derived stats for you
*In app copies of the rulebooks that you can copy and paste from to character sheets or monster/npc sheets.
*Importable 3d models of minis and a 3d terrain system, with the option to flatten to 2d and tokens. Hexes and squares/cubes. Effects on the terrain, like flickering light sources, winds, fire...
*Shareable Music and audio (unlikely to be legal, sadly).
*chat feature
*The ability to show notes, images etc, and to pass them to selected participants without everyone getting them.
*Video chat
*PC, Mac, and Linux support
*Possibly support for speech to text like Dragon Naturally speaking to allow for rapid adventure generation, or recording transcripts of the game.
Basically, everything Fantasy Grounds 2 does, and more. More flexibility for other systems, and more power.