Last name plus sensei.
To me, first name plus sensei sounds very odd. It's like calling someone Mr. Bob or Ms. Mary. I don't think people have done that for quite a few decades...
If someone deserves the title "sensei" to you, then how can you also be on a level with them where you feel comfortable calling them by their first name?
A democratic Aikido dojo is also an interesting concept. If you have removed something as fundamental as the sensei - student, sempai - kohai relationships, is there anything still Japanese about your practice? Or has it been completely stripped of its origin and Americanized?