This can be used to make multiple stories i will talk about all of them further in.
------points of interest------
MC can be both male (Daud, Corvo) or female (Emily, Billie Lurk) all of them have combat training and outsider's mark. You can even pull ROB reicnarnation story if you want.
Power levels aren't that wide apart.
All of MC are or were assasins at some point. They are quite strong fighters who have experienced fighting supernatural (witches, ghost dogs, etc.)
Story can go in many different ways from enjoyment (Pesonaly don't think that harem is viable here) to kingdom building and all out war
MC has certain kind of depth to them Corvo has a daughter and dead wife, Emily is an Empress who was forcefully dethroned already twice. And I don't know much about Daud, but he had an entire organization of trained assasins with power.
Outsider mark looks kinda like mark on Esdeath.
You can easily incorporate both world's together since AKG emtpire is costal nation (picure in coment) and Dishonored Empire is multiple island nations banded together. So you just have to put them on the mostly empty sea and it will work.
MC can be on both sides of conflict or join as a thrid party. Since Dishonored empire is quite a place authoritarian (kinda), armed police patrols, coruption, difference between rich/poor. You can make MC kinda good guy, antihero kind of guy (with viliany conduct), or straight up bad person especialy considering Daud. Another thing is that they would easily be able to ally with AGK empire since they are quite simmilar in conduct.
You have a lot of possible romance options be it male MC x female, female MC x male, Yuri or even Yaoi (don't know how you would pull that off). But please remember character backstories Corvo has a daughter and dead wife.
Another thing to think about is technology advances from side of Dishonored and AKG. While AGK have tengu and all that, Dishonored have ark pilons, light gates and power of whale oil. They're cities are more....Steampunk especialy Dunwall. Then there are the clockwork solidgeres.