With the Duel Decks: Heroes vs Monsters revealed we can talk about the two mechanics previewed for Theros.
[Cost]: Monstrosity [Number]. (If this creature isn’t monstrous, put [Number] +1/+1 counters on it and it becomes monstrous.)
Heroic – Whenever you cast a spell that targets [CARDNAME], [Effect].
What we’ve got here is a creature based mechanic and a spell based one (even though it doesn’t seem like it). Both of these have their little quirks and I’m sure I might get some of the smaller details wrong without an official FAQ. But I can tell you some things that we can be on the lookout for with both of these mechanics and what we might be able to expect in the block. I’m taking a large assumption that these will be spread out over the five colors though obviously there will be only-color effects.