It should be interesting to see who he marries or if he marries. If he does, it is more than likely to be someone of equal blood, like some of the princesses mentioned, than not. If he does not marry at all, then that line of Sucession is over. So it is rather imperative that he does. As for someone of unequal blood, he might want to, but I think he would be steered away from that, as his is the only Branch of the former Romanov dynasty that is still fully royal by the laws of the house. An unequal marriage would harm that. Anyway, it certainly is difficult to find a suitable marriage partner in this day and age, but even more so when you are royalty with so many requirements of a future wife. As for Prince William, he is more free to marry whom he likes.