In regards to By The Grace of God being a single, not gonna happen. Much like Not Like the Movies wasn't a single, and much like how I'm Still Breathing wasn't a single.
Don't get me wrong, I absolutly love By The Grace of God, and think it's an amazing song, but no one is A) Going to play it on a popular music station for 1) It's more depressing elements and 2) It's major slowdown compared to other songs such as Roar or Dark Horse. B) Going to want to listen to it on a regular basis (minus katycats) like Roar, Firework etc.
The song is best off being left on the album and on tour.
Moving on to the acoustic album talks.
Full on album is not bond to happen for awhile, if at all. Once she's done with PWT, people are going to be hungry for more new Katy Perry. They're not going to want to go from songs like Birthday and Roar to acoustic songs only. Maybe an EP or something, but a full blown 13 track album is highly unlikely to happen.
Moving on again, she's not going to "leave" pop. Katy Perry is pop. It's not like she's going to do a 180 and become Kat Perry and go all hardcore metal (https://www.youtube.com/watch?v=eIzoWNQqnEQ
As for which album is the real Katy, they're all the real Katy. Like it was said previously, they each showcase and are made up of Katy at that particular moment.
Katy Hudson was back when she was really into her faith in terms of hardcore christian or w/e as she grew up with pastors for parents.
OOTB was made up of her own personal writings as she tried to make it big.
Teenage Dream was big on love and more cartoony and fun loving as she was finally in the big light.
PRISM is all about love and self-empowerment and shows the matured side of Katy.
Katy is a textbook case of a good musician. Evolving every step of the way. You always want your second album to be better than the first, and the third better than the second etc. etc.
Lets take AC/DC for example. I kinda like their music, but notice how a lot of their songs seem like they could be off of the same album. Why do you think I love KISS so much as well as Katy? KISS has 20 albums, and each one sounds different than the previous. Katy has 3 (not counting Hudson era), and each one has a different and unique sound.
Also don't expect to see a Katy/David song anytime soon for the same reasons listed above in Kenobi's post.
I stand by my extreme want and desire for a box set or something D:
Assuming they legally can, include Katy Hudson, all three Perry albums, demos of songs, alternate versions of songs, previously unreleased songs, music videos, lil goodies etc.