Ask HN: Customizable Desktop Messenger?

Are there any highly customizable desktop messengers either cross-platform (maybe even a web app) or for mac?

Right now I use Signal which is great but it's missing a lot of features I'd like, many of which are so specific to me it wouldn't even make sense to contribute them via PR. I want to be able to transform the messages directly, add new buttons, toolbars, etc.

I'm looking for a kind of chrome-extensions-for-messenger type thing, ideally not tied up with some kind of social media, or ad-network, or gaming, or with a bunch of business features (like Slack etc.). Just a vanilla, extensible, desktop messenger.

If worse comes to worst I could whip one up for me and my comrades but maybe y'all have a link that could save me some time.