RArakoo·Nov 13, 2014
You must be aware that notmuch is only the search backend, and for a full MUA you need frontends. I believe the official one is the Emacs one, so it should be fairly usable. There's also a web one [0] you should be able to play with.
Another contender in the space (label- and search-centric) is mu and its ui, mu4e [1]. Something else to have in mind.
Now if you want something that works on desktop and mobile, something worth a look would be using SQLite and its built-in full-text search... see how far you can go with that. SQLite is available pretty much everywhere, Android even allows full-text search. Now what you have to do is synchronize SQLite dbs. It "shouldn't" be too hard to remove old emails from the db so you can keep X MB worth of it. You can even shoehorn it into couchbase-lite [3][4] so that sync is automagically taken care o
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JMAP looks cool (definitely more interesting to implement than IMAP) but it seems to be more a query API than a sync API, although there are facilities to "get changes since last time" (a HUGE improvement from IMAP as deployed everywhere). OTOH if you can shoehorn it into couchbase-lite, you can use a generic sync protocol that can be used for other things too (caldav/carddav).
Heck, you've piqued my curiosity, I see something doable here. I'd love to hear more. I might even hack something just for fun.
[0] https://bitbucket.org/wuzzeb/notmuch-web
[1] http://www.djcbsoftware.nl/code/mu/mu4e.html
[2] https://github.com/couchbase/couchbase-lite-android
[3] https://github.com/couchbase/couchbase-lite-ios