Ice Beam would be an amazing coverage move for Keldeo.
These, off the top of my head, are the current OU pokes that resist Keldeo's STAB options:
That's a total of 10 pokes. Keep in mind that not all of them are solid Keldeo checks or counters - I don't think that Starmie would enjoy switching in on a rain boosted Specs Hydro Pump, for example. The addition of Ice Beam would grant Keldeo super effective coverage on 60% of its current OU STAB resistors, in addition to providing neutral coverage on Gyarados.
New possibilities aside, I think that Keldeo will unfortunately kick OU back into a more weather dominated metagame, as we saw in its early days. A rain boosted specs hydro pump is just that powerful, and we haven't had a truly potent OU abuser before this point. Choice Band Terrakion already makes short work of a lot of "physically bulky" OU pokes. Keldeo is the special version of that, but has the added benefit of gaining a huge boost when it is raining, at the cost of less effective STABs. I think it is potent enough to shape the metagame to some degree, and turn OU away from its current non-weather majority, because weather makes all the difference both using and battling against Keldeo.
EDIT: I suppose Toxicroak is also a STAB resistor due to Dry Skin, forgot about that.