Here are some kits that I think would do very well for Rubicon models. Not only because such vehicles were common in and would be popular among gamers/modellers, but more importantly because no one else makes any of the below in a plastic model kit... yet.
SdKfz 10 Halftrack
Panzer II
M3 Medium Tank (Lee/Grant)
Matilda II
SU-76
T-70 or T-60
BT-7
A few Allied AT guns - currently they only have German AT-Guns.
I think Rubicon's M4 Medium (Sherman) line of kits sell well, and their planned Panzer IV and Panzer III line might also sell well, But when their American, British, German, and Soviet vehicle products are more complete, maybe Rubicon can look into French, Italian, and Japanese vehicles. However, is there a good market for French, Italian, and Japanese vehicle model kits?