I know the mercenary pool is already bloated, but there really are a quite that could be in the game that represent areas lacking in mercenaries or units that lack a variant. Some of these units could also be made natives, espeically the Miquelet
Sekban (Turkish, Mercenary Janissary)
Székely (Hungarian, Uhlan (this is the old Hakapelit, basically)
Geschwindstück (German, Light Cannon)
Genoese Crossbowman (Pavisie Crossbow)
Mitrailleuse (Organ Gun/Gatling Gun)
Lisowczyk/Harcownik (Horse Archer)
Border Reiver (?)
Ruga-ruga (Gasgenya)
Gossains (Indian shock infantry)
Purbiya (Sepoy)
Méhariste (Sowar)
Seimeni (Serb, Musketeer of some kind)
Saika-Ikki (Ashigaru)
Bargi/Pindari (Oprichnik)
Mounted/Horse Grenadier
Chapelgorris
Bashkir/Kalmyk (Qizilbash)
Frame Gun (mercenary Leather cannon, based off Scot Conventor armies.)
Registered Cossack (Piechur)
Miquelet (skirmisher)
Legionnaire (French Foreign Legion)
Oštrostrijelac (Sharpshooter)
Amusette (Abus Gun)