I went the total random route for thematic emphasis. I can see how that makes at little too confusing and unpredictable to be easily used though, so I have no idea allowing for a bit of control in what creature gets targeted.
Basing it off the opponents mark certainly fits the theme. I somewhat dislike that it would allow it to be cast without any creatures in play in the opponents side, but I like how it shifts the meaning so it is interesting at least.
I first envisioned having a spell that generated a rival for some creature your opponent had in play. Shifting to base off opponents mark makes it more like generating or recruiting a creature that is specifically your opponents rival.
I will definitely shift the element selection to be a little more clear cut, but I would like a little more feedback on which option seems best.
1) Player picks an enemy creature
2) Based solely off of opponent's mark
3) Player may choose either an enemy creature or their opponent (mark).
All three options have merit and drawbacks so a little more feedback on what people think is most useful / balanced would be good.