Bloodcast has the undesirable effect that you may end up not being able to cast the spell if you don't deal enough damage. The spell would then end up being exiled permanently, a la Suspend. Not sure I want an easy possibility for that to occur at common.

Yeah, these cards just don't work. The trigger is very easy to prevent, with the forewarning you get as a defender. Even if it were reformatted to remove the forewarning, Bloodcast just isn't working. Maybe Bloodcast should be ditched for a different mechanic.

The idea behind the mechanic is very flavourful for the set, its just the current execution that doesn't work. Why not simplify and make Bloodcast an alternate cost similar to evoke and the Zendikar traps, etc.

Bloodcast : If you dealt combat damage to an opponent this turn, you may cast ~ for its Bloodcast cost instead of its normal cost.

I figure 4 mana for a 4/4 Trampler is a pretty good deal and it would encourage attacking which seems to be one of the keys parts of the set. Also, you don't have to mess around with exiling cards, targetting specific players, or worrying about dealing enough damage to be able to cast the card.

That's an excellent idea! It kind of combines Bloodthirst with an alternate cost.