The author of the Mutt E-Mail Client put it like this: "All mail clients suck.". I agree with this statement.
I'd be interested in an email client that embraces the following aspects:
Journaled IMAP operations, so that large bulk operations are far less likely to fail. This way, even application crashes, reboots etc. wouldn't affect the bulk operation. I haven't seen an email client so far that is capable of moving a lot of messages (~100k) at all…
This taken as the first milestone, would enrich the open source landscape with an robust IMAP migration application.
Implementation-wise, I'd prefer a high-level language - preferably Python. The use of pypy right from the start can probably boost the applications performance. I'd use a lightweight ORM (e.g. peewee) and a proven storage solution (e.g. SQLite). Alternatively, a document-based data store like Blitz-DB or ZODB might be worth to consider.
Of course, to have a full-blown email client we'd need support for HTML messages, filtering, GPG, CalDAV, CardDav, …
It could be named "ovenmail - the over engineered mail client".