Little late to the party, but my first pitch would fall into the "optional" category.
As I've said before, I had an idea of a "dreamworld" scenario in which the heroes are transported to either retrieve something or put an end to a curse-like spell.
The caveat is that I thought about this revolving around Conan, so tell me if you'd like us to keep with this or if I should change the main character idea so that we may more easily negotiate things with him down the line. I'm going to use some of the points you've made before, as well as some of the KS content as I feel it makes sense in the circumstances (but I could tweak it so that it goes with pretty much any setup we like once we set it in a timeline) so it goes like this (and I may be taking some lore-related liberties):
Hadrathus finally gains an audience with King Conan, but between bureaucracy and other events overlapping, the heroes can't get a direct audience with him.
Instead, Pallantides approaches them with a request on Conan's behalf.
It seems the King has heard of Thoth-Amon's threat, and would gladly jump at the opportunity to stretch his sinews and lash out at the necromancer. But there are others who think it wiser to pursue a tangential issue first... The King has been having premonition-like nightmares for the better part of a few months, not long after Hadrathus first found out about Thoth-Amon's plans.
In them, Conan does face-off against this foe alongside a band of adventurers, but he dies while simultaneously taking Thoth-Amon down as well.
While Conan himself takes no issue in this, real or not as the premonitions may be, there are others who feel it wiser to take all stepts for the crown to remain upon his head for a while longer.
It has come to the attention of Pallantides and others that there may be a way to ensure the nightmares never come to pass, but a complicated ritual must take place, and it involves transferring the essence of those involved into another plane of existence to "kill" the nightmarish apparition itself. There is a contact that Pallantides has made at a local inn that is vital to the cause, so they must travel there first in order to perform the ritual and transferrence.
Here's where it gets tricky, and stop me if I go too far.
I want to blend 2 maps into one scenario (tavern and fortress) and have both of them go on at the same time.
The idea is to split the party (Don't I know it...) and have half (or so) accompanied by some Black Dragons remain in the taver and face off against some regular troops, while the rest goes over into the "dreamworld", and fights alongside Conan (the core box Conan or Conan the Wanderer, in any case one of the variants of him as a free spirit rather than a King dealing with kingly minutiae) to take on the apparition of Thoth-Amon and his clique of demons and spirits, of which there are enough in the Core to have this proxied if need be.
There are lots of balancing issues and fiddling, I agree, but this could be truly something else and something worthy of such a glorious arc as we have planned.
The only setup issue would be the Overlord dealing with action in 2 spots at once as the heroes will be split anyway and only focus on one half of things anyway.
Should they win, Conan gets a powerful boon in the final battle (maybe a healing spell or a reroll-to-end-all-rerolls, jury's still out).
Should they fail, Thoth-Amon will benefit from either an event that the OL can trigger against Conan, an increased damage output against him, or our barbie will simply suffer from a lessening of his abilities overall in this final fight so that he faces an increased risk of being defeated.
Heroes:
Hadrathus
Pallantides
Black Dragons x5
Yogah
*pepper liberally with your other choices so that we may keep this consistent*
Debating whether to add some more "mercenary" options for adventurers who seek to gain Conan's favour and pledge themselves to aid in this endeavour.
Opponents:
Tavern: Guards x5 led by a single villain character
Fortress: Skeletons, Mummies, Bone Golem, Forest Demon, GIant Snake, Thog, T-A's apparition... Not all at once, of course, just laying out some possibilities.
I know it's a lot to take in, but the concept has me chomping at the bit to see it unfurl, so lend me your C&C.