There are certain Pokemon that can't or won't be added for a variety of different reasons, such as balancing and whatnot.
What if you could complete a small quest or defeat the Pokemon and "befriend" it. This would allow you to befriend the Pokemon and "equip" it as an additional follower.
I love Mew, but I understand why he wouldn't be added to a game like this. I would absolutely settle and be happy with the idea of getting him as a follower as part of a quest to befriend him or something like that.
Thoughts?
