I think successor-war armies would be great, including a broad variety, e.g.

Macedonian (Phillip V)
Seleucids (Antiochus III)
Ptolomy
same states in asia minor
greek cities (Syrakus, Rhodos, ...


different type of cavalry
different type of infantry (greek, macedonian, asian etc.)
war elephants

there are really a lot of gaps for ancient and medieval things.

I think a lot of ancient-greek fans would need better indian (Maurya) Sets against Alexanders Macedonians. Also medieval Rajput or Mughal Indians would be very nice, because they are missing until now, but that's another Theme (I thought on Figures using the Chakram, the Pata/Gauntlet-Sword and the Katar-Dagger and so on). New Indians would furthermore give a better army than the existing Sets and fit the Elephants better.

Also Huns and Avars are rare or not existing, even if you can put them together out of many horsemen-sets, but then the army is a bit motionless.
I also asked about camp- and marching greeks/makedonians, as they exist for romans, in the far past.

Another Point is, that there is no armoured war elephant without a turret. They also don't use flail-balls or the tusk-swords active - they just stand around. Maybe that will be fine, so an assault-scene will be possible. As an Addition, swivel-guns could be added to an elephant set, so one set can be used also for medieval to „modern“ indian armies.

I agree with the comments about Sumerians, Assyrians/Babylonians etc., Palmyra, Etruscan, but especially the eastern Cultures are less attentioned, as i can see.

If i'm allowed to add: For medieval knights aren't very much mail-armed horses with full and half mail-coats, i think.
And: While there are the most important early american native nations on the market, there aren't any civil figures for this cultures and also Amazonas-Tupi or Karibs ect. don't exist, as well as mounted Arquebusiers with the capacete/morion-helmet, which can be used widely.
To get mor "ancient" again, what's about minoic or mycenean figures or something "mythological" like Atlanteans?

Sorry, i had a long time to think about, because mostly modern sets arriving the market, which isn't mine.

How about the Roman siege engines for Masada? Pretty big though..