Put me down for #2.
A lot of people are not sophisticated enough to set up "proper" 1v1 games against AIs. They wouldn't know which AI to use or what settings or which maps give the best gameplay against AI. Putting it into the ladder would give them fast access to different AIs and teach them the names of AIs and such.
So adding AIs to the ladder for some players would help them to have a broader gameplay experience. But limit it for example to players under 700 rating. Anyone with over 700 rating should be able to set up their own custom games IF they want to play against an AI.
This would also help to gauge how good the AIs are (constant feedback in terms of what "rating" the AI would have if it was human) and give replays to review to see how to make AIs better
It would give more diverse experiences to lower-rated ladder players
People generally are less stressed out about playing against AI, so even just the possibility that they might draw an AI opponent might make the ladder a little less scary and painful for people without a lot of ladder experience
Making a new queue is bad. That would just complicate things. More options to players is not more better. It's better to improve the options they have, where you can.
"If you don't like playing against AIs, then just git gud" -- I think it's fair to say this. Lower-rated players usually don't even hate playing against AIs. What low-rated player would feel punished if they searched ladder and got an AI opponent?