Possibility.

Fourteenth vs Allen, to whom does glory belong? And how it would affect his relationship?

Cross predicted that Allen will eventually be the 14th himself. However, Allen insisted that he would always be himself. For so many times, he had tried and succeeded. Until when?

Allen almost lost his self when the 14th announced his purpose to the Earl, but he managed to stay being himself. The Earl then created a condition which allowed him to give Allen a tempting offer: turn into the 14th to stop the killing frenzy. Seeing how many victims there were, Allen was ready to give himself, but everyone opposed to it. That was how the latest chapter went, so no, please don't ask me what would happen next. But if he really turned into the 14th, he would have a great impact to his relationship with the Order members.

Cross's words about the possibility that "Allen would have to kill someone important to him" were significant. The words he picked could be translated in many ways--it could be one person, or some people, or everyone. Obviously though, the someone he was referring must come from the Black Order.

Considering how Lenalee and Allen's connected dream also had a link to the 14th, I think if Allen were to turn into 14th, he might have something to do with Lenalee as well. She could be the one he'd end up killing, yes. Along with some other people, probably. Lenalee actually had noticed Allen's changes, ever since Cross told him about his connection with the 14th. When she was in Allen's room, she also saw his different "self", but she decided to keep it to herself because it might have been her imagination, because she wanted to trust Allen. I think she should have thought of it as an early warning.

However, I believe that Lenalee's definition of the World would somehow change Allen back; but that's just me. Again, we could only wait until the mastermind reveals everything to us!