So there are plenty of "niche" Napoleonic troop types not represented in plastic. But I think I've identified a genuine MAJOR gap.
"Hungarian" Austrian Infantry. Austria was one of the major players in the Napolenic Wars. And while there are Perry and Victrix plastic kits representing "German" (effectively any troops not Hungarian in that large and diverse empire) troops the other roughly half of the Austrian army is not represented at all.
It's interesting to note that Austria's opponents considered the Hungarian troops to be fiercer than the "German" elements. Whenever an Austrian element was engaged it was described as "Hungarian" as there was more credit in it.
What are the differences between "German" and "Hungarian" Austrian troops?
("German" - Ignore the headgear for now.)
(Hungarian - Again ignore the headgear for now.)
The chief differences are "Germans" wear gators while the Hungarians wear low boots, and that the Hungarians have braid on their trousers.
Back to headgear....
The Austrian army went from "kasketts":
Over the course of the war.
If there's room on the sprue, than all three types of headgear would be welcome. If it has to be narrowed down the helmets and shakos would probably be most useful.
If one includes bearskin heads the kit can make Hungarian Grenadiers as well:
(The Perry "German" Austrian kit includes helmets, shakos, and bearskins. Victrix sells 3 seperate boxes (Infantry with helmets, Infantry with shakos, and Grenadiers) that all have the same bodies, but different head sprues.)