Board Thread:Manga/@comment-27321453-20180708204808/@comment-27823881-20180710035010

I've been giving this some thought, and I think there might be another way for Eren to use the "Coordinate" ability on his own without the need for someone of Royal Blood:

By acquiring all remaining Titan Powers so that he carries all 9, essentially becoming Ymir Firtz's equal.

I think the reason why only direct descendents of Ymir Fritz could fully utilize the "Coordinate" ability while all the other Titan Powers could be utilized by any bloodline was due to the fact that she possessed all 9 Titan powers, granting her "Royalty" status. From what we know for sure about the history of the world, all 9 Titan powers being combined in the same host has never happened again since. Unfortunetly, this would require Eren eating Armin and Annie, but considering how dark the series has gotten recently, it wouldn't surprise me if it happened.