Most definatly. I'd hope that Luigi and Daisy would work alongside one another to save the Sarasaland. It'd be a fun twist and allow for two player co-op (or competitive!) play. It'd be a nice way to get more than one "real", as many put it, Mario came in a system.
But, you're probably right about Eclair. Still, maybe Peach will reference her in another Mario RPG. Something like "I was having tea with Daisy the other day when Eclair called..." or whatever. Come to think of it, what DOES Peach do when Daisy visits?
No sig ideas...yet.