I'll go light...
The first double album in Hip Hop was started by DJ Jazyy Jeff and The Fresh Prince with 'he's the Dj Im the rapper" also The concept of the double album existed way back in the 60's. In the 90's Prince and Smashing Pumpkins were already doing it before Pac. But I'll give it to Pac for promoting it as such in hip hop.
GRODT was the first manifested hype of mixtape popularity...We never saw that in hip hop. 50 was supposed to be finished. He Re invented himself by first changing the mixtape game as we know it and then making good on his hype with his Shady Records debut. GRODT was the final phaze of the underground mixtape hype. Plus aint no gangster rapper (while alive ) ever have the crossover success as 50 did with GRODT. GRODT changed the game because it was the end result of mixtape hype which is what we use to follow new artists till this day. Its a hard act to follow and only few have been able to do it.
I know we talking hip hop but when i think of game changing concept albums I think of Marvin gayes"whats Going On" Pink Floyds The Wall....someone said 'prince of theives' started the story arc from front to back. But De Las 2nd album kept its story theme from beginning to end. But if someone feels thats debatable.... Cubes, Death Certificate as a concept album with The Death Side on side 1 and The Life side on side 2. "the death side, a mirror image of where we are today, the life side, a vision of where we needed to go" The funeral in the beginning..The death at the end of side 1..The birth at the beginning of side 2. The final breakdown of whole concept of the album on the song "Us" follwed by No Vaseline at the end of side 2. Straight brilliance. I'll even throw in Cuban Linx being that Rza said it was designed to be like a movie. Then after that was Kool Keiths Doctor Octogon album were the same them rides from beginning to end.