I don't dare comment on upcoming card abilities, though I can safely say that the four keyword abilities from Ravnica will not be on any future guilds--each guild gets its own keyword.
As for future guilds, though, we can chat all we want. The best hint we have so far is this page:
http://www.wizards.com/default.asp?x=mtgcom/arcana/934 , which gives us the guild logos for all of the remaining guilds. The next set is actually the second one listed here, Guildpact, and it looks like its three guild logos are a sunburst with a circle in the middle of it, a big dragon, and burning tree. The burning tree is easy enough to figure out (green/red), and it's almost guaranteed that the dragon is blue/red; that leaves black white for the sunburst, which makes perfect sense--it's probably a solar eclipse.
The symbols for Dissension, the third set, include a burning skull, a wavy tree, and a circle with three triangles around it; these aren't as immediately obvious, but we can figure them out just by elimination. The burning skull is of course black/red, the wavy tree is probably green/blue, and the only thing left for the triangle thingy is blue white.
So now we know (or are almost certain) which guilds will be in which sets. Any ideas about what each guild will focus on?