I think most of his buttons are apparent and you have free reign over, but also:

What if Sidereal is just a dream? Or what if Subaru gets ripped away from Sidereal and sent back to his own world, to Empty Tokyo. He hasn't accepted it fully yet, but he wants to be in Sidereal where there can be Something Else with all these people.

I wouldn't mind an opportunity to use the sakura tree (eta: and eye) as an anchor/token too since it's tied to Seishirou, plus the tree doesn't like the Lonely encroaching on its guardian. What if Sakurazukamori Subaru...

Available for full offense if convenient, since they're in the maboroshi the sakura powers.