Just out of curiosity, since Telegram’s client is open source, couldn’t someone in theory fork it and then modify it so that it’s actually using XMPP but with a practically identical UI? I understand the changes would not be trivial, as you’d probably have to replace/rewrite a ton of code, but maybe there’s something in there worth using?

Aside from this, you have a few projects already to look at. Android has Conversations, which is so good that Android doesn’t actually need another client. If something truly bugs you about Conversations you can fork it, as many have done, or maybe a change you propose could be accepted upstream.

On the PC there is Dino, which so far is my favorite. It is still missing features and needs improvement, but what they’ve done so far is very good. You might want to contribute to that first before writing your own client. It would be best if Dino were ported to other OS’s, but somebody somewhere is working on that I think.

I also noticed a brand new project called Kaidan which might be the sort of thing you’re after, but it’s in such early stages that it’s not really usable.

Out of all platforms, it seems Windows users are hurt the most badly by really only having Gajim available. Gajim is usable and has many features, but in my opinion it’s just not good enough.

You could also check out Movim and see if that works for you. It’s probably the best web interface you can get for XMPP.