Hey bro :)
Have you no theme(s) for the jam? This is not cool :\... If you want to host a jam, first you need is a theme, or an structure for suggest and vote for themes, but never start a jam without a theme... This leave participants sad and discouraged.
Now, if you want some themes, check out the GM48 Jam (gm48.net), Ludum Dare (ldjam.com) and/or some "game idea generators" (like Orteil's - http://orteil.dashnet.org/gamegen) available on the web. They will help you choosing a theme for the jam. But PLEASE don't select an veeery strange/surreal/supernatural theme and, after choosing one, post it here (if possible, before 2pm, so devs can start developing their games as soon as possible).
Platforms I think they should not be specific, letting developers to make for her preferred platform (Android, HTML5, Windows, Linux, etc). But if you want to specific one or more platforms for standardize where the games will run, I recommend using the two or three most popular in the world, which are Windows, Android and HTML.
So now, let's make the jam cool :D
#SorryForMyEnglish #Brazil :P