That Xanatos Gambit as he reffered to occured to me aswell, Kyros would succesfully unite tiersmen and also other realms against the threat of a new overlord.
Kyros offers a chance to get rid of the Edicts, but who is to say that the new Overlord will?
but whilst i very much agree, about the tonal shift...cause there is a huge one, you go from Servant to Master in a relative short time.
It felt like a HUGE payoff, ...but a payoff i really couldn't enjoy, due to the game's abrupt ending.
I'd have Kyros lay siege to all spires, and making them unavailible, and the act 4 would be about either invading the north, or amass all armies and factions to unite against Kyros forces, or kill off the factions to prevent them interfering with you.
And this time around when you reclaim the spires, it wouldn't be oldwall battles again (perhaps find a way to use the bane against Kyros forces inside the oldwalls but nothing more.), it'd be more like Army VS Army (they could use the conquest mechanic again maybe and then let the battles play out against them, and depending on your numbers, tactics and loyalty you might be succesful, or pay a heavy price for 1 spire)
Let's say the Sunset spire.
You can use Disfavored to crush Kyros forces with A/B/C attack modes and tactics, and you'd have like..a counter of your army size, and if you make bad choices or have a small army or low loyalty you will have some hardships reclaiming the spire, perhaps you'd need to negotiate with old enemies, or go in yourself and clean up.
and if your loyalty is lacking in perhaps Bronze Brotherhood or beastmen, they'd unite with Kyros to kill off your forces unless you deal with them first.
etc etc etc, just top of my head stuff.
and in the end you would reclaim the mountain spire, depending on your amry size, your future rule and ending will be affected.
if your army is close to extinct that would mean Kyros forces retreat, but would later return to kill you.
if your army is huge, then you will establish your rule as Overlord over the tiers and south, and Kyros would either let you be, or you could unite in an agreement, or you'd march on the north.
if your army is...medium sized, that means you might have to come to either an agreement with Kyros, or surrender, or try to hold, but your rule wouldn't be as enforced giving you a more gray ending.
and all these endings would have you meet with Kyros on the top of the spire for some last dialouges and choices that affects the ending.