The answer with the smaller vehicles is more moulded-on detail rather than more parts. Rubicon are already very good at this, as evidenced by the level of detail on the tracks/running gear in recent kits like the M5A1, JagdPzr 38t and Crusader. Because early war tanks had narrow tracks, you'd generally be able to do their tracks/running gear in one piece. Headlights etc could be moulded on - I think both wargamers and modellers would be fine with some degree of simplification if it meant getting plastic kits of rarer vehicles like early war French, British and Soviet tanks. However, I would guess from previous discussions that the fact that many earlier AFVs were riveted is a bit of a problem when it comes to maintaining a low parts count.
One thing Rubicon could consider is providing 2-in-1 kits - a tank and an armoured car or softskin in one box. Companies like Airfix, Nitto and Matchbox did this in 1/76 scale, and these sets were very attractive. Throw in a couple of figures, and you have something that will appeal to wargamers and modellers. So, for instance, you could pair up a Somua S35 with a Panhard armoured car, a Panzer III Ausf E with a Horch Kfz 15, or an A9 Cruiser with a Dingo scout car - that kind of thing.