I would pay for a lightweight and featureful Matrix client like mIRC is for IRC.
Currently Matrix clients seem to be split into two groups: featureful but slow and bloated (Element and forks), or completely minimalist (copying popular messenger apps, very little customization, or TUI-only). None support scripting and few support theming.
I realize this would be a huge undertaking, especially since the protocol is constantly evolving, but maybe it would be a way to continue mIRC's legacy? I would hope that a lot of the experience that went in creating mIRC would apply. Well, at least I hope Khaled is considering it!
