LOL! This is now full circle to, “gee, why don’t you @name mention the person in a reply if you want them to look at it?”
Note that email invites to a topic (for people who do not yet have an account) do already include the topic title and a summary of the first post.
For regular notification invites on existing users, there’s no place in the notification UI to enter a lot of extra text, so that’s not in the cards. The topic title will have to do.