There are also features some consider good UX but Signal shuns because they create confusion regarding the security properties (i.e. self-destructing messages). Signal's number one priority is security while that is not nearly as important for Telegram in practice. A lot of the UX advantages the latter has can be traced back to this essential difference.
I agree though that something like messaging multiple people without creating a group first is more or less UX-only and would probably make sense (groups are anyway a client-side abstraction only in Signal, servers have no concept of them, so making anonymous or automatically named groups on the fly should be doable)
Also probably getting in touch with OWS on their mailing list is more effective than hoping they notice random posts on HN. It is not the most contributor-friendly project on the planet but clearly articulated and focused posts are usually answered and so are issues/pull-requests that conform to their stated guidelines (do not add extra options, do not put security second, etc.)