Comet is in the fall, which aligns to when the new raid drops. You should probably temper your hopes and dreams now.
Still stand where I did earlier today, when this was a suspected but not confirmed turn of events: a proper survival / arena mode is more appealing to me than another raid, at this right now. Key word is proper: it needs variety in progression, a good meaty environment (or set of them), modifiers that rotate, etc. If it's a liner progression of rounds with no variation Prison of Elders will disappoint even those like myself who would welcome it. I'm optimistic, but we'll know soon enough.