This really isn't possible as it would pretty much undermine their lore as far as shanks are just grunts that follow the higher ups. The only thing I don't know is if the Prime's are the top of the chain. As far as I know the Prime's are practically religious figures to the fallen houses, I'm not sure if they are higher than the Kell's though. Thats really the only options. Either the House of Wolve's Kell is not actually dead and will appear in the House of Wolve's dlc and it'll start a giant rebellion in the Reef where all the HoW fallen are subservient to the queen right now and the raid would be you taking down the Kell.

It'll definitely be the archon priest on steroids...

I have a sneaky suspicion it will be a larger/shinier version of an enemy we have already seen too many times before

Randal the vandal should be the final boss, but in a spider tank...and after you disable it he jumps out to make repairs with 2 arms while the other 2 shoot at you, and you only have so long before he is able to hop back in and continue the fight, and you must repeat this until Randal's health reaches zero (his health should be independent to the spider tank's)!

Sounds like the death star lol

Someone said giant archon priest, with a sepiks prime head, riding a walker, launching stealth vandals at u.

The final boss for the fallen has been extensively discussed and the community concluded that it would be Randal the Vandal, riding a spider Walker, with a massive pet shank.

Well, you did the impossible and that makes you mighty.