I found this thread because I was googling "28mm plastic miniatures", so obviously for me the answer is yes. I like ancients, so Victrix and Warlord have both been good to me. With one of the most popular ancients games being Hail Caesar right now, which requires something like 200-300 models a side, 28mm is a very smart move for you to make. I will echo what others have said and encourage you to make your models compatible with other ranges - after all, the only point of marking scale in the first place is to know which dudes work with the ones your friend has.
While there are several other companies making plastic 28mm models (Warlord, Victrix, Perry, Gripping Beast, and Conquest to name a few), there are still huge gaps in the market in my humble opinion. Look around at people playing Hail Caesar and Kings of War Historical on Youtube. It seems to be all armies which are available from Warlord for $0.75 a model - that is, Romans, Celts, or Greeks with not much else. I have a suspicion that if other sets were available at the same cost, we'd be seeing them as well. I would encourage you to ask people interested in 28mm gaming what armies they feel aren't represented. I grabbed a list of all of the armies covered by Kings of War historical - Hail Caesar has far, far more. Many of them have no inexpensive plastics available, which means your competition is metal figs coming in at maybe $2 a model.
Anglo-Saxons - Avoid. Gripping Beast and Warlord are having a race to the bottom with pricing, and Victrix is coming out with a much higher quality set in 2019.
Aztecs - no plastics that I've seen, but no one is clamoring for them either.
Barbarians - Celts are cheap as dirt from Warlord, and Victrix has several sets as well. Archers are missing though, so that's an opportunity.
Byzantines - Gripping Beast is working this angle with their Late Romans, and Fireforge has more expensive later Byzantines.
Carthaginians - It's Victrix or nothing here, but they have a quality range at a reasonable price.
Chinese - None that I've seen, but I've heard a lot of interest. Opportunity here for sure.
Conquistadors - No dedicated plastic ranges but I imagine some of the Pike and Shotte stuff from Warlord would work for most of it.
Crusaders - Fireforge is hitting this angle hard with their Deus Vult sets. Some of the cheapest plastic cavalry around. Avoid.
Egyptians - Nothing, and a lot of call for it. I will buy 100-200 figs on release day if you come out with a good 28mm set here. Warlord has the cheapest I've seen at about 2 bucks a fig for infantry, and $50 for 3 chariots. A Hail Caesar army would be at least $500, and a Kings of War one wouldn't be much less.
English - The Perry's have their stuff but that's about it.
Franks - Nothing as far as I recall, but Gripping Beast has lots of cheap general stuff for this era.
French - Again with the Perry's and not much else.
Greek - Nope, Warlord and Victrix have this area WELL covered for cheap.
Holy Roman Empire - Iirc this covers Pike and Shotte era HRE, so Warlord probably has it covered for the most part.
Huns - I've not seen anything, but I'm confident that there are armies out there that are actually made up of Fireforge Mongols. Could work out.
Indian - Nothing here in plastic world. As a side note, this army as well as the Egyptians and several others allow elephants at 3-6 models a unit. The cheapest manned elephants in 28mm are currently $15-20 from Gripping Beast and Victrix, so myself and many others will happily buy some cheaper ones.
Japanese - Warlord has a cheap box here that people like. Good luck getting in on that action.
Macedonians & Successors - Warlord and Victrix both cover this.
Mongols - Again with the cheap Fireforge models. They'll be tough to beat here.
Normans - Conquest has hilariously cheap models here.
Ottomans - Not sure. I think Warlord has some models here, but there's not a great deal - not sure how compatible the arabic models from Gripping beast would be.
Persians - Wargames Factory had a set which wasn't super highly regarded, but Warlord (who bought them out) doesn't carry plastic Persians. Who will all of those cheap Greek models fight? Big opportunity here! Just wait for me to paint up all of those Egyptians and you've got me here too.
Polish - No plastics as far as I know, but no one cares.
Romans - Warlord's flagship Hail Caesar set, and Victrix has them as well. With that said, depending on the cost of Victrix's upcoming set (probably ~$35-40 for 16 models), a low cost auxiliary cavalry set might do well.
Scottish - Mostly standard late middle age fighters which can be covered by Perry and Gripping Beast. There are no "Braveheart" style ones though if you want to bend reality a bit.
Slavs - Fireforge has one plastic box, but it's relatively expensive.
Spartans - If these were any cheaper from Warlord they would be paying you to take them.
Scythians - I haven't seen anything here. A box of horse archers would probably do well.
Thracians - The army list itself is bizarre, with falxmen led by Spartacus, so I'm honestly not sure what they were going for. Between Victrix peltasts and Warlord falxmen, I guess this is covered?
Vikings - As with Saxons, Warlord and Gripping Beast are having a bit of a price war here.
Mainly, I have to say, don't necessarily look at what armies people ARE playing (90% Celts vs Romans), look at what armies people wish they were playing. The entire Bronze age is neglected, including Egypt, as are the Persians that everyone would love to pit against their Greeks. Alternatively, if you do look at what armies people are playing, find what isn't available or is still expensive. Celt chariots are still $15 each (I don't expect the upcoming Victrix set to change that), so are elephants as mentioned above, cataphracts seem to all be metal and $4-5 a model. Artillery tends to be expensive as well, apart from the Roman Scorpions that Warlord includes in their starter set and legionary box.
I didn't mention later periods as I know far less about them, but unless you go very niche-y, you will be directly competing with the big boys. People LOVE their Perry and Victrix models, and generally don't paint them fast enough to kill their pockets. With that said, if you can produce high quality sets that match standard 28mm models and halve the cost, then as stated above, it will be down to distribution/people actually knowing about the things.
Apologies for writing a book. I wish you the best of luck and I'll be keeping an eye on your line.
When Hat first started making 28mm figures, although I do not collect them, I thought that at least it would mean an increase in quality in order to compete with the likes of the figures produced by the Perry´s etc.. However, with the number of new sets being produced each year dropping considerably any new scales that would slow this down even further would be worrying. I also wonder if the collectors of 28mm hard plastic miniatures also consider the soft plastic used by Hat. As to sales I imagine that Hat must have some idea since they would know how well their Penninsular War British Heavy and Light Dragoons sold. So they would know if this scale could be a money winner for them.