That would be very nice to see. At least let them do some more stuff and let us hear about that stuff. In WHFB, I always felt they were extremely neglected, though at least not as extremely as some other factions. And even though a live Kroak would be absolutely epic, I prefer it the way it is. An ancient, cosmic entity whose power bleeds through the mummified remains of his long-dead body, guiding his army and people against their old enemy, while displaying the unfathomable sway of a god in face of the Chaos Gods themselves, their beings and presence not able to simply ignore his indomitable will, as they can most coporeal beings. Him being alive again could take away the awesomeness he provides, being a cosmic force, coming forth as a fraction of what it truly is, as the power bleeds through its dead vessel. Rarely seen in action but when it is, it is truly awesome and stellar.
yeah dead Kroak seems more appropriate. Plus the whole "im literally too powerfull to die even if you destroy my physical body" is a nice difference with the regular undead or otherwise ressurected heroes. Making him ressurect seems like hed be demoted to "merely" a regular godlike being.
A cool way to include him in the lore could be: All of Sigmar’s Alliance end up having to join together for one epic battle with Chaos akin to the Allpoints or something similar. All the armies are fighting and it ends up boiling down to just the leaders of each faction duking it out with the major Daemons of each Chaos god (eg Alarielle and Volturnos fighting Rotigus, Gordrakk vs Khorne greater Daemon etc), and the Celestant-Prime and Nagash fighting Archaon. The battles all end with just-about victories for the forces of Order, however, Archaon ultimately defeats all of them. All seems lost and Chao’s victory seems absolute when all of sudden, cue Kroak, who then proceeds to annihilate all the Chaos forces at once (Archaon’s mortally wounded form being whisked away because story wise the Chaos gods aren’t done with him and actually because there’s no way GW are killing their most expensive model). Absolute victory for Order, a time skip can occur to bring in new events and fluff settings, and Kroak gets his long overdue moment of badassery.
Kroak is definitely the most powerful spellcaster in Warhammer fluff-wise, however, it is my belief that Nagash is the second most powerful spellcaster, easily.