If they were to implement this (they won't) then they'd definitely have to finally allow us to delete chats as potentially people could be getting dozens of messages asking them to join guilds. And it's already bad enough having a huge list of dead chats with long gone players you can't wipe.
Also, a lot of people (including myself) have the setting where only friends can message you, so you'd not be able to message a lot of folks anyway. And there's no way they should remove that option as players should always have a choice if they want spam in their inbox or not.
The best solution would just be an invite to guild button - with a small line of text you add yourself, much like the add friend button. Players would then have the simple choice to click the tick or smash the x button to remove the prompt. You should also only be allowed to invite someone once, if they refuse you shouldn't be able to keep spamming them (see Clash of clans for this annoyance)