As a HUGE fan of everything Soviet, I'd say there are fourkits that would be awesome to have, in decreasing order of necessity.
1) Late War Soviets. M43 uniforms, with wide shoulder boards and standing collars. Warlord's offering only has early war M35 kit, and Wargames Factory's offering is... bleh. Rigid poses, grossly underdetailed weapons (I've seen 20mm stuff that looked better), wildly incorrect shovels, and extremely flat details like rifle ammunition pouches and tent halves.
That's why we badly need a good looking set of plastic Late War Soviets. PPS-43s (nobody does these!), correct equipment, and dynamic poses displaying some variation, like jackboots and low boots with puttees. Massive bonus points for Polish rogatywka hats to make East Front Poles, and/or Kalashnikovs and RPDs for 50s-60s Soviets.
2) Soviet Cavalry. Regular uniform bodies, standard Soviet (side cap, helmet) and Cossack heads, so that you can do both usual and Cossack units.
The only ways to do WW2 Red Cavalry right now are a) flimsy 3D printed Cossacks that break when you look at them the wrong way, b) extremely expensive metal Mongols or Russian Civil War cavalrymen that you have to touch up, or c) long conversions of ACW cavalry that require you to chop up two boxes' worth of troops. Plastic Soviet cavalry would be a dream come true.
3) Eastern European Partisans. Some bearded guys, some winter caps, some women, maybe a teenager, and a more rustic look than the French Partisans with their fancy scarves. Perfect not only for Soviet partisans, but also for the Yugoslavs - both Tito's and Chetniks with the right heads - and even for Poles in Warsaw if you throw in a pair of heads in German Stahlhelms.
4) Soviet support weapon teams. Basically what PSC tried to do, but better. (to their defense, their heavy weapon teams are scaled up 15mm stuff, and look fine in that scale). More variation in poses would be needed, so that you don't end up with clones. Maybe more weapon options, too ? Like a SG-43 machine gun, an Ampulomet or things like that.