I've been toying with this thought for a long time. There is no online game based on FreeSpace, in fact there aren't many good Sci-fi online games altogether. I am not saying i'm doing this, but if i found the time, what should the game look like?
RPG (simple gameplay, but fast availability):
Very simple, you'd have your own ship you can further customize(swap and upgrade weapons, crew, ...) with choice of different classes (warship, miner, transport, ...) and specific set of single player missions. As you level up, better ships will become available to you. Main goal is accumulating experience, single currency, improving your ship and participating in coop and PvP missions.
Travian type RTS (annoying long waits gameplay):
Every action in the game will require certain amount of time to complete, like building ships etc. You have only one planet you can build on and gather resources from. As you build your fleet, you'll be able to engage in SP and coop missions and PvP. the game would have more complexity and be more challenging to mind than RPG.
Strategy (complex and challenging gameplay):
You would be able to rule over more planets or even star systems with huge army, controlling almost every aspect of your species. This can either be realized as turn based game (you get turns, or action points over time, multiple actions (ie building, training troops,...) can be done in a single turn, but all actions require certain amount of turns to be spent) or recount type game( much of the actions is done instantly(building, training), some of them in real time (transport) and some would progress in certain amount of time (battles in a war, resource mining)
If you have an idea for another system, i'm open to suggestions. Also, if sufficient number of PHP coders, artists and people with several other skills (flash and action script guy, server and network guy, some other guy i forgot to mention...) offered assistance, work might begin immediately.
If you have any other question, ask below and also feel free to offer help if you wish.