I've found myself thinking about this a lot lately and while being able to play Vassal solely on a mobile device would be great, with the recent developments with table style displays like the Microsoft Surface, the feature I'm most interested in is being able to use one or more mobile devices as a companion to a larger screen. Essentially the large display would be where the game board is, and is where the game is played, while each player could use a mobile device to keep all of the pieces he/she owns that are not on the board for whatever reason.
I'm not asking for Vassal to require mobile devices of course, and I'm sure that this would be possible on our end even if you didn't do anything special code wise, but full, integrated support for this style of play would be amazing. Only a couple of things would be necessary for this kind of support as far as I can tell:
1: The ability to view the game board in fullscreen mode, making buttons and the chat window as unobtrusive as possible, while still being accessible
2: The ability for the player to view their hand in fullscreen mode
3: The ability for multiple devices linked to a single player to connect to a server so the player could use their computer screen for the game board and their mobile device to manage their hand if they wish while playing online.
4: The ability to have at least one device connect to the game in an "open" mode giving that device the ability to interact with any piece on the table and put a piece in any player's hand, but not necessarily giving that device vision of the contents of all players hands (this is to allow multiple people to play sitting around the same fullscreen game board while using their mobile devices to manage their individual hands)
Thinking about it, 3 and 4 may already be possible, but I haven't used Vassal a whole lot and have never tested to see if they are, so forgive me if they are.