I've read through all 10 pages and the various email app suggestions. I'll throw my two cents in.
What is with the lack of spell-check in android? I know - predictive text and multiple keyboards... but we all make mistakes from time to time. For business users that are blackberry converts, the ability to spell check before hiring send was a great feature. Can this not be integrated in an email app? !?
In my fantasy email app - the delete from phone or from phone and server option on the blackberry was awesome. Something you need to save, by don't want to clutter your inbox? Delete from phone only. Don't need it at all? Delete from both. That feature doesn't exist in android. Yes, k9 and droidmail have the delete from phone only setting, but not the option per individual email.
Search function within email would be great.
You build this and you'll have a winner. Just add spell check and it'll make a killing on the market. I am still floored it isn't part of the native email app.