Is there any way we could change the demon illusion to take some sort of skill or level into place? 31 minutes at mastery is nice, but its a dead end with no room to grow.
For instance, what about (Level - 25) x 20 seconds? Thats another 25 minutes at cap. I suggest 25 because thats when you learn 725, but it could be done any way. Demonology ranks, Sorcerer spell ranks, and of course, level.
Additional guild skill ranks! Every rank in another guild skill adds 10 seconds!
Just some ideas. I'd really like to see the duration increase in some way.