For those interested, here are some of the more interesting rumors about the upcoming Magic Block (ie, the next three sets).
The names are verified: Onslaught, Legions, and Scourge (in that order).
The Cycling mechanic will be back, with the same name and same effect.
The main new mechanic (which has been pretty firmly verified) is one called Morph, which lets you play a creature face down (as a token, basically) as a 2/2 for 3 mana. None of the rumors I've read mention color, so I'm assuming that the 2/2 creature is colorless. Then, at any time, you can pay the Morph cost for a facedown creature and flip it over; it then becomes a real creature. One example I've seen is the Blazing Firecat, which Morphs for two red mana into a 7/1 creature. I don't know for sure if you can play the creatures straight from your hand, or if you have to bring them out unmorphed and then morph them. Either way, it sounds like a very interesting mechanic.
The second new mechanic is one that has not been fully explained or confirmed. It's called Tribal, and has a coat-of-arms kind of effect (ie, Tribal creatures get bigger when similar creatures are in play). This is a lot like the old Plague Rats ability, which I always loved, so I'd like to see this rumor turn out to be true.
The final rumor is a weird one, but one I find to be extremely intriguing. You know how in Odyssey Block the colors were unbalanced from set to set? In Onslaught Block the colors will be even, but the spell types will be unbalanced: Legions allegedly contains nothing but creature spells, and Scourge will have no creatures at all. This, of course, is still an unproven rumor, but it's very interesting.
All in all, I'm think it sounds like an interesting environment. The cycling in particular will interact well with Threshhold and Odyssey Block. What are your thoughts?