I honestly feel like Demo will play similar to how it is on live, minus the "awkward mechanics" (Hand of Gul'dan double stacks, Molten Core, etc.) and with a summon demons core mechanic instead of Meta stance dancing. I can see Demonic Fury being replaced with Fel Energy (same as our pets have, except for us it's built up manually), which will then be used to fuel summoning situational Guardian uber demons which burn through your Fel Energy much like Meta used to Fury. Basically, what we do in Meta on live will now be done by greater demon proxy. (Soul Fire/Demonbolt single target damage? Summon X. Chaos Wave aoe/cleave burst? Summon Y. Nether Plating or other Meta utility? Summon Z. Pet twist to your heart's desire, as long as you have the Fel Energy to sustain the summon.)
Along a more radical line of thought, I could see the concept of greater Guardian demons made Demonology exclusive as part of its new core mechanic, with Affliction and Destruction single target/aoe damage being adjusted for the loss of the Infernal and Doomguard.
However what I don't believe is that Demo's new army of demons will be that vast, as to not have the spec suffer from having too many damage sources. For that reason I could see Demonic Calling and Imp Swarm being removed, and Doom becoming a caster form spell with its crits being the only source of Wild Imps summons along with maybe Hand of Gul'dan.