Gitrog monster is most likely the most powerful combo piece spoiled so far. Off the top of my head:
any self mill card will gain card draw
Fetching will allow card draw
Draw for every kotreliquary activation
Magmatic insight= discard a land draw 3 for 1
And my favorite... borborygmos enraged.
Bobo will just kill your opponent. Gives every land in hand "cycle to lightning bolt your opponent"