That's just a tiny bit too "wide open", don't you think? I mean, the chances are pretty good that you'll want to maintain some sort of involvement in the site once you've coded beta version 0.1 and made it public, right? It can be the coolest thing ever, but if it's not something you're personally interested in, then it's going to die on the vine as soon as you're finished.
"Highly interactive" doesn't tell us much, either -- that can mean anything from something like Google docs (real-time, multi-user word processing and spreadsheets) or an online whiteboard to a drag-and-drop music composer to a chat to a MMORPG.
So you need to tell us -- what turns your crank?
Most successful sites/apps do one of two things -- they either scratch an itch, or they help you get laid. (Sorry for the crudity, but it's the truth -- and the secret motivation behind Facebook. The idea of turning it into money came much later.) Let's concentrate on the itch-scratching part if we can. What do you need that's missing from this world? Is there anything that you are tired of explaining over and over again that could be easily visualized in a web app? If you're thinking in terms of a game-type scenario (it doesn't need to be a "real game"), what would you enjoy messing around with?
Think it over for a while. You'll either come up with an idea, or at least narrow the field down enough that you can start asking for opinions on a number of different ideas. The alternative is to do the equivalent of free development of someone else's "brilliant, can't lose, instant billionaire" idea -- and if you want to do that, just register with one of the online contracting sites, where you'll get no end of useless, uninteresting and impractical ideas to work with (and make nearly three dollars an hour while doing it).
