I think it's a good idea. As mediocre as 40k Demons are...that's how awesome Fantasy Demons are...except for Demonettes...they're meh in both worlds. The only thing that's tough with Demons is fitting in some of the nastier Lord choices. Thanks to the percentages in 8th, it's not easy to fit a Greater Demon with more then a few tricks. Of course that's where Heralds come in, but the Greater Demons are just so much killiness it's tough to go without.
I was trying to figure that out myself. It seems like the percentages become a pain with the big monsters. Honestly though, I was thinking a very core based army. None of the rares or specials really appealed to me. So, in my mind, Heralds and their cores sound awesome.