How do you plan to add alt text, via local image processing?
Also, to support the use case of rich text formatting, the transformation must support changing the type of the output from plain text to HTML, so additional metadata or API would already be involved.
I would hope you planned to confirm sending following typo autocorrection also, so that would need some further API or logic.
I could also envision slash-like commands in the same box like how certain messengers implement it for common actions, such as switching chats, changing your per-chat display name, avatar, changing the group name, kicking, /me (prepending your name to the message) /shrug (and other sideways emoji shorthands), sending in a named sticker, producing an invite link to your other groups, etc..
What could preferably be only done through live editing is autocompletion of the name of participants.
Spell checking could first highlight all your errors after initial submission and ask for confirmation or redrafting, but that’s a bit clumsier than if it was done live.