Tag co-op mode is fun! There are maps that are tailored for a certain number of players to co-op, such as TAG2/4/etc...
However, if the number of players doesn't match the requirement, the map will become nasty and difficult to play. (Recall the corner jumps in the Knights of night)
So I come up with the idea that you can add Auto as a player in Tag co-op mode to fill up the requirement. So even if you gather friends and end up only 3 people, you can still enjoy TAG4 map.
A possible way may be: The host can right click on an empty slot, and Auto will occupy that slot. You can kick Auto just like normal player. You cannot pass host to Auto. Something like that.
However, if the number of players doesn't match the requirement, the map will become nasty and difficult to play. (Recall the corner jumps in the Knights of night)
So I come up with the idea that you can add Auto as a player in Tag co-op mode to fill up the requirement. So even if you gather friends and end up only 3 people, you can still enjoy TAG4 map.
A possible way may be: The host can right click on an empty slot, and Auto will occupy that slot. You can kick Auto just like normal player. You cannot pass host to Auto. Something like that.
SPOILER
In fact, I wanted to implement a bot that accept invitation to an MP of TAP co-op, and quit after beatmap starts. So it will be replaced by Auto. But I am sooooooooo fear peppy will ban me because of multi. 
So I gave up.
So I gave up.