Mikado, emperor, an ally?
Maybe...
Princess kaguya...
Elixer...
The emperor burned the elixer, and never got to see kaguya-hime again.
Miyuki already said, if he had to, he'd live forever, to wait for her.
I hope this is a subtle hint that mikado (who could easily be a candidate for onii-kisama's plan) will not be a threat, but as a shijo: a friend? Also he seems afraid of kaguya anyway, so he might not go for such a plan (would he fake cooperation for a friend though? Long enough for kaguya to take power or uncover a weakness in the shinomiya group?)
Am i being unrealistically optimistic? Maybe... BUT LET ME HAVE HOPE DAMN IT!