Hi Timo,
I would also, like others, see you mainly working on Dovecot as an IMAP server. As far as I can see there are many things on the roadmap, and I hope many more will be added (for example a built-in health-checker for director backends).
Only if you have enough personal resources and Dovecot as an IMAP server will not "loose your attention", I would love to see your expertise in making a better MTA.
You are talking about bigger ISP installations, and there you always have at least 3 tiers: Internet-facing SMTP servers, in-the-middle-SMTP-servers delivering local mail to Dovecot via LDA or LMTP, and some outbound SMTP servers. For these middle-SMTP-servers that more or less just connect to Dovecot to deliver local mails I could see a more lightweight MTA solution, so instead of having Postfix+Dovecot I would like to see Dovecot(+MTA features) only.
I'm not sure if I would use your MTA as the Internet-facing server, where "just" a fast SMTP server is needed with good Spam filters, Anti-DDOS-Features and so on. But that would be the position where all your strict DNS and TLS features are needed. I would love making email more secure by default.
I totally like your idea of the object storage instead of local files for queues. That is an awesome feature for situations where your harddisks fails, your postfix-server burns down or goes into long maintenance. Having mails in a more central (redundant) place is very cool, so if one server dies another can quickly take over all "his" mails. That feature is awesome for the outbound SMTP servers, where millions of mails are "stored" in the queues for many days, a harddisk failure is a big problem there.
Sum up: I would love to see you working on a MTA, but ONLY if you don't neglect the worlds best IMAP server :-)
Michael