I am suggesting what is in the title, where you would be confined to a small area, and either at a constant rate, increasingly higher rates, or increments (waves) enemies may spawn, and come after you. in a waves system, there could be for example wave 1, with some amount of money and the ships are randomly selected in budget, or predefined as in wave 1, (enemy 1) (enemy 2) or whatever. also, Upgrading! I was thinking that every 10 or something waves, you can have a second ship, and every wave (or enemy killed) you get money.
Survival/King of the hill mode
It seems like a great idea. I think that procedural waves would be best so people don't learn the patterns, or have it as an option. It also allows for a more exciting time seeing random ships and having to try to survive them. The money idea is also great so you can have extra ships or repair current ones.
I've been thinking about something along these lines where if you survive a wave, your ship then gets uploaded to the server for other players to fight when they play survival mode. Would make fun co-op too!
Added to trello as "under consideration": https://trello.com/c/JUMsaJuF/182-survival-mode