No offense, but all the tuta* variations and keemail.me don't seem really appealing to me. I think you do a wonderful work and I'd even consider paying for your service, but the names aren't at all appealing... Something more low-profile, maybe shorter could attract more people without requiring the users to create a custom domain.
We do not plan to add more domains at the moment. If you want a different domain than one of the currently available, you may upgrade to Premium.
Thank you very much for your support.
It's $10 a year to buy your own domain. Why not just buy exactly what you want?