Exactly what i proposed before. Excellent idea.
I'll try to help.
Btw, what with Ottomans as playable faction ? Or atleast with Tatars ?
Same question with mercenaries..... It was a lot of German, Scottish and Swedish units in "magnats" personal armies.
Also, i believe, that some of existing equipment from vanila and other mods can be used with permission of makers. Berdiches, halberds, pikes ( mainly for mercenaries infantry ), some chainmails for "towarzysz pancerny", shields, bows (reflexive) and guns.
About units tree, i would like to see "Lisowczycy" as well
I'll try to help as much as i can. My art and scripts skills sucks, but I'll do my best.