What you really need is somewhere with something more like a discussion forum, than this Q&A site, also that place needs to already have an active community of developers hanging around and actively contributing.
I don't think that this sort of thing is relevant here, but it is certainly something that another website could (or perhaps already does) do. I think you'd have trouble getting a new site up and running to do this, as it will be very easy to attract people with ideas ()perhaps even demands) for new apps, but quite hard to attract the necessary experienced and motivated developers. You'd need a site that already has a big enough community of devs with spare time on their hands to bootstrap your idea.
Having said all that, there have been a couple of times lately when someone asked for an app that doesn't exist, and a member or two here have gone off and written one (this one springs to mind), but that is very rare simply because most people here are Android Users, rather than Android Developers. StackOverflow does have the developer community, but requests for new apps would definitely be off-topic there.
XDA is one place that could do this, but currently doesn't - I think they do allow requests for new custom ROMS and changes to existing ones, particularly for models that haven't got one yet, but as they don't specialize in app development they may not be willing to expand in that direction.
