This is probably the biggest ask of all time and not possible unless Delta Chat gets more funding so don’t take it too seriously compared to some of the lower-hanging asks, but:
It would be great if Delta Chat was also an XMPP client so I could have OMEMO messages and email in the same app.
Hi Sandra, I am not sure if mixing this two types of ecosystems is a good idea? For me it is fine to just use two different apps for both. But this is only my point of view!
I just think it’s such as shlep to use XMPP 
We are collaborating with Cheogram who have recently added webxdc support so there is that. I tend to concur with @tapete that having separate apps has its virtues. Adding XMPP support to our core Rust library would be feasible i guess (update: check @link2xt comment below, he knows better), but an extra sizable effort as you noted. But i am not sure how the UX of xmpp messengers maps to Delta Chat’s UX … roughly they are similar but the devil is in the details, typically, and usually underestimated by folks not involved with implementing UX/UI. FWIW about half or more of DC’s development effort is around UX/UI work.
Groupchats work completely differently in XMPP, they are hosted by a single server and most operations on them are not available offline.
OMEMO would not be possible to add to the core because OMEMO specification boils down to using libsignal which is currently strictly AGPLv3-licensed. This would require that we distribute the core under AGPL-v3 which is too much of a restriction for bot developers and distributing the app via Apple App Store.
No, it is not necessary to support XMPP. What about Skype and ICQ protocols?
Skype at this point is some HTTP-based API, rapidly changing and breaking backwards compatibility, not worth trying to build an alternative client.
Modern ICQ is just a brand reused for an yet another messenger, but there is an alternative open source implementation of the old protocol IServerd still hosted at http://kicq.ru/ that can be used with many old ICQ clients.
If you are interested in multi-protocol clients, maybe it is actually easier to install Pidgin and revive development of Delta Chat plugin for it purple-plugin-delta. OpenSSL issues stated on the readme are no longer relevant if you build with Delta Chat core >= 1.122.0 since it has switched to Apache-licensed OpenSSL 3.0.
Skype/ICQ was irony. Delta chat is IMAP chat and somebody suggests XMPP support.
The background for this post is that I was talking to someone and I said that I’d long been daydreaming about a double ratchet scheme for email but recently found out that that’s not gonna work “since email is asynchronous” (
) and then the guy said “what about an app that had an email-like interface but used XMPP (with XEP-0384 probably better known as OMEMO, and XEP-0201, Threads)?” and I thought OK the best email interface is Delta Chat but that’s probably not gonna happen within my lifetime and there are lower hanging asks I want more (like setting non-autocrypt keys & avatars for non-Delta Chat-users) but I might as well start the thread since threads on this Discourse is like issue tracking but for feature requests and then I started the thread. I’ve been a long time user of multi-apps like bitlbee. Wasn’t happy to get mocked as some sorta ICQ-user. 
This sounds very similar to the wording of old version of Delta Chat FAQ which has just been updated: FAQ - Delta Chat
There is a link to a prototype of Double Ratchet on top of OpenPGP.