About multiplayer, i think a simple 1vs1 is good, but i would want also a "container": something like a place where players meets and create/join games (like battle.net and the like), maybe with a mechanism for trading items, units, etc.
What facilities do you have to implement this? Do you have an environment that you could run Delphi ISPAI DLLs or even a Delphi TCP Server?

The options are
Easiest: An ASP site that is accessed through HTTP calls. Then returns data as the output of the ASP sites (see my web live tutorials). Requires an IIS server.
Harder: A delphi TCP server. Accessed from clients using IdTCPClient. Requires a Windows server where you can run your own executables.

For my DogFight entry I'm going to do the ASP option. But thats because I have an ASP based web site, but cannot run executables.