This is really up to you but if you need any ideas, here are some:

 'goodbye muse!'

'Muse music start!'

'Good luck Muse!'

'Nico-nico-nii~'

'Harasho'

'Fight with me!'

'Thank you muse'

'nyya!'

'Kotori...What???'

'Do your best'

'*insert charcter name* Godbye!'