it would be nice to have CAPTCHA support in mod_register, so you can activate In-band account registration and don't have to worry about bots. One might say this is not necessary because of mod_register_web, but for normal users it's much handier to went through the whole registration process in one client, instead of open a web registration form first over a browser. Also the argument that clients don't support it, is a bit outdated these days since a lot of them (Gajim, Conversations, Pidgin etc.) support it already. Also ejabberd had implemented this feature into it's mod_register.
I looked into the mod_captcha_registration before I wrote this, but it seems outdated and I don't feel comfortable by replacing components of the core, with something which is not really serviced.
It would be nice to have this to make the service more attractive for the normal user. I would even give a bug bounty for this.
trashserver.net speaking here. I'd like to see support for captchas in in-band registration, too. Like imo, I've disabled in-band registrations because of bots, but my users would like to take advantage of the more comfortable in-band registration. Captchas are already supported e.g. by Ejabberd and many clients, so I'd really like to see support for that in Prosody.