[@MrDidact] 
[hider=Dances with Dragon's spoilers]
It's actually super interesting the only person who's getting stronger as a greenseer/warg is Bran who has been a cannibal since he passed the wall ending in the last book where he ate the other greenseer in his group.

Euron also uses a large amount of blood sacrifice in his magic or it could just be for fun. 

[/hider]