Imperial Navy Thunderbolt Fighter- The Valkyrie/Vendetta are cool and all, but some sort of Strike Fighter capacity would be excellent for the Guard. This is a classic model and very easy to make in plastic IMO. Could potentially be made available to all Armies of the Imperium. A workhorse of the skies that should be used in more games. I have a Forgeworld one I need to refinish. If they came out with plastic ones, I'd buy 2 more no questions asked and run a flyer wing for sure.
Space Marine Storm Eagle/Fire Raptor- Another 30k/40k crossover potential. While a plastic Thunderhawk would be amazing, it is probably never going to happen (a Titan would happen first). BUT This is pretty close. It can me made into 3 different units (Fire Raptor, Storm Eagle and ROC pattern storm eagle) and is a great looking model to boot that already uses several plastic Storm Raven Parts. Plus, the usual warping people have to deal with with the model would be a thing of the past.
Ork Squiggoth- Orks don't get enough love, especially since the Stompa is one of the worst LoW for its points in the game. A brand new, more dynamic sculpt along with some new cool rules and you will see it fly off the shelves for sure. A usable Gargantuan creature with endless conversion possibilities on it. I guarantee you everyone who plays 40k would love to see this, whether they play orks or not. Given that Orks also have no true answer to the Knight, this would be a smart move IMO.
Tyranid Heirodule- Another beasty for the game. If anyone should have more plastic gargantuans it is Tyranids, and the Heirodule perfectly fits the bill. It is the right size, playable in normal games and comes in two flavours. New rules once again and you'd have something highly effective and desired by players, especially if it is made to be their answer to Imperial Knights.
Imperial Guard (DKK, Elysians, etc.)- This should follow the Horus Heresy model of late. But it would be really awesome to get 1-2 core sets of various guard regiments that I can they upgrade with Forgeworld parts or kitbashing. Everyone LOVES DKK, but they are the most cost prohibitive army in the game. Having a plastic infantry box of them would be amazing, add variety, and have many many uses (IG, Chaos, Renegades)
Necron Pylon- YOU MUST CONSTRUCT ADDITIONAL PYLONS. Another Xenos force without a lot of attention recently. This kit is super simple and yet has 3 profiles I believe! Really great potential for a cross over to plastic and something that I am sure a lot of Necron players would love to see. Given that IG has like 70 versions of the baneblade and Imperial now have Knights, there is no reason why Crons can't have another LoW, particularly one in the price range of Knights.
Eldar Hornet- I keep seeing these. A lot. They just are really popular right now. Really that is the extent to my logic of making them plastic. They are a fun little kit with some good rules. Index it in price right between a Vyper and a Wave Serpent and I sure it would sell in droves.
Space Marine/Imperial Guard/Solar Auxilia/Renegade Rapier Batteries- Why isn't this a thing already? Seriously, it is one chassis design that can mount like 54 different weapons to it. It is usable in 30k and 40k. It is usable by SM, IG and Renegades. I mean COME ON! It screams "MAKE ME IN PLASTIC". Heck, you could even combo in the thunderfire cannon with it if you tried hard enough. It is such a useful kit, but so expensive right now. In plastic it would fly off the shelves and has great magnetization potential for sure. The only problem would be "What crew do we sell it with?"
Space Marine/Imperial Guard/Solar Auxilia/Renegade Malcador- Last but not least, yet another 30k to 40k vehicle, And unlike the rapiers, the crew problem is really easy- just put 1 space marine on the sprue and 1 human with 3 heads (1 IG, 1 Solar Aux and 1 chaos). BAM, you have a "Warhammer 40,000" generic kit. Its numerous profiles in 40k and options in 30k make it a great combo kit once again. Plus it just looks plain cool.