Say that Nicholas II dies soon, we can't have Alexei since he would die before or very close after adulthood
Mikhail is over-used (No offence, he's in a lot of TLs)
The only successor I can see is Sergei Alexandrovich (If he survives his assassination and wasn't a total @$$hole to jews, but then again, everybody was pretty much mean to jews back then.....)
Only else that is out there for succession?