There would have to be contracts in place, etc. Otherwise there's huge liability where some disenfranchised forumite who proposed an idea (that was used) could create legal issues
That's a risk even if there's no money involved, unfortunately. If this gains momentum, better to have a sign-off form granting perpetual non-exclusive use to the game project of whatever stuff people come up with (I don't think actual copyright transfer is necessary, but I Am Not A Lawyer, and even if I were, I'm not from the country most likely to produce vexatious litigants).
So the caveat there would be game engines. RPGmaker is definitely not my first choice, but it is the lowest barrier to entry. And then we have to assume we have people who can use that engine.
That's kind of cart-before-the-horse at this point. Figure out the gameplay basics first, then find an engine that will accomodate them as easily as possible. (Frex, let's say someone thinks it's cool to inject platformer elements--how easy is it to make RPGMaker handle those, or would a more generalized engine be better? How about paper-doll sprites? Are so many animations needed that it would be better to switch to 3D? etc etc ad nauseum.) There are other engines with visual editors for non-programmers, although they tend to be more generalized.
There are Mac emulator-type programs to run RPGMaker games as well
No Linux ones for most versions of the engine, though, unless things have improved over the past two or three years. And even if there were there's no guarantee they would work. (I've had closed-source games fail on Linux for reasons ranging from "segmentation fault" to "developer forgot to mention that a certain piece of garbage called pulseaudio was required, and apulse appears not to be good enough". As such, I'm rather jaded about the supposed uptick in Linux games.)
(I'm also strung out on painkillers most of the time at this time of year, so adjust the value of anything I say accordingly.)
