Myrmidian witch hunters seems like an odd concept, since she is a goddess of war, strategy and such. Morr, Ulric and, of course, Sigmar I can see witchhunters of, the latter especially. But Myrmidia? Curious why you chose that road?
In any event, I'm curious to see the results. I know I have hit a point where I want a little of different things and can't stick to my Riekland theme, so I have created contingents from realms my general has holdings or that he has business. Marienburg is one contingent I am starting to have fun with. Or I will once I get some more units made.
I'm sure you can find a way to integrate your existing Marienburg units to whatever new form your army takes.