I will be honest, I haven't came around to using XMPP myself yet, but I am pretty sure you are able to do all of those things, at least with the help of extensions. Here's the XMPP standards that are currently stable or experimental:

Files/Images:

• XEP-0363: HTTP File Upload [Stable]
• XEP-0446: File metadata element [Experimental]
• XEP-0447: Stateless file sharing [Experimental]
• XEP-0448: Encryption for stateless file sharing [Experimental]

Emojis/Stickers:

• XEP-0444: Message Reactions [Experimental]
• XEP-0449: Stickers [Experimental]

Looking at clients, I see various ones that have at least some of the features you are looking for (especially file and image sharing, I haven't found a client that has explicitly implemented the stickers specification yet, but Movim seems to have them):

• Converse.js
• Dino
• Kaidan
• Movim
• Profanity

I would give it some more time. It seems those features are in development, but this is essentially an alternative to Matrix that is more standardized. It's a widely adopted and integrated protocol, so I don't doubt the features you are looking for will become more fleshed out some day.