Slavic factions in history rarely found success with super-heavy units in the way that, for example, France did with the Medieval Knight, so I think it would be more fitting for a futuristic Polish faction to be lightly-armoured but fast moving and able to hit and run in an extremely irritating fashion for anyone trying to fight them. Medieval Poland had an unusual quantity of horse archers for a European nation.
Also if there were to be Death Fields Lithuanians, it would be great to see them embody the spirit of the Pagan Duchy of Lithuania that fought heroically against the brutal campaign of religious extermination conducted by the Teutonic Knights, as each Death Fields team has to be based on a particular respectful cultural archetype (as the Bulldogs were based on Zulu War British, Les Grognards were based on Napoleon's Old Guard, e.t.c). Perhaps include units championing different Lithuanian gods like those in Medieval 2: Total War's Teutonic Campaign.
Giltine's Chosen - elite infantry with polearms (which would look even cooler in sci-fi armour):

Followers of Perkunas - lightly-armed monk-like infantry with scythe-like polearms (a unique melee weapon option even in sci-fi):

Dievas Guard - cavalry armed with long fork spears (which could be electrified in a sci-fi version):

Of course a Death Fields equivalent would have to have more futuristic tech and so on, but certainly just like the Bulldogs, Einherjar, Grognards and Raumjager still bear a resemblance to historical British, Viking, French and German units, a Death Fields Lithuanian unit could still draw upon these Medieval Lithuanian units, and the gods of Baltic Paganism they worshipped, for inspiration.