I think a great addition to this application would be a system to allow PLAY BY EMAIL. Most virtual table top websites like BoardGameArena have system like this, so you can play games, but take your turns as time allows, nor have to meet up online in realtime. One of the most common forms of this, that people have been doing long before the internet was playing chess by mail.
Now I know technically you can email someone your save game, and this kinda works... I make a move, save it and the email it... they load it, make a move and email it back. Still it is fiddly and not very interactive as well as being open to abuse.
What might be cool is a way for TTS to record a log of the game in progress and the ability to "play it back". This could even be a entire feature.. you could play your wargame and then "watch the reply" like in RTS games and see all the actions.
Anyway, having something like that so you could receive your PBM data, and then "watch" them take their move. Would be awesome.
You could add a way to enter email information directly into the app so TSS could send and receive mail. Click END TURN and it mails it, click START TURN and it receives it.
i realise this is a major suggestion and probably not going to happen, and if it did people that are actually problem solvers and programmers could think of a way better way to do it... but the basic suggestion is...
TL;DR
Would be great to have some kind of system for playing a game in a PLAY BE EMAIL style, where you can take turns over days or weeks.