Hmm. One idea to differentiate is to go the way that Starcraft races are balanced (this is a huge simplification). The basic Starcraft units - zergling, marine, and zealot are (again grossly simplified) designed so that 1 zealot = 2 marines = 4 zerglings.
We already kind of do this with the Norse being way weaker than the the Greek units, but bear in mind that Starcraft ONLY has three races, so they can do that more easily. We are planning to add lots more civilizations, and having them all be unique just by stats alone really wouldn't work. I think the differences need to be in behavior or special abilities. There's a finite number of things we can do there as well, but if you look at (say) AI War, I think it's a pretty cool model in terms of a lot of the crazy things that become possible.
I think the differentiation is more fun when it's an obvious difference, so I think it's important that differentiation doesn't focus too much on what it gets a bonus against. I think most players can't be bothered to hover around and check what "type" each unit is and compare the different bonuses.
I think you are right, but I think that also the differences of those bonuses is also a good thing. It's just a staple of strategy games, and some people really do like it.
To give more flexibility, you could balance out huge advantages with huge weaknesses. For example, beserker units could have a property that makes them take 2-3x as much damage from a counter attack, and a huge penalty to their own counter attack, but they could get a huge attack value to make them effective shock troops.
Yeah, that's a very good point.
EDIT: Another idea is to give some units some kinda auto-cast "cooldown" abilities. As an example, Longbowmen could have some type of "rain of fire" that blankets a 4x4 area with attacks for the turn, but they can only use it every 5 turns.
Ooh! This is a pretty cool idea. That would be a great thing for expansion. There is one problem with that, though: if the AI isn't absolutely perfect at what you think it should do, that will be hair-tearingly frustrating. Taking that example:
1. Does the longbowman fire at a weaker target in the middle of a bunch of other targets? If so, the targeting logic gets very specific to him, and very challenging.
2. Does the longbowman hold his fire in terms of that ability if there are not suitable targets all in one place? If so, then that's really challenging to know when to use it, because the situation changes all the time. And since he's intentionally not closing distance to enemies past his minimum firing range, that's going to mean he's rarely in a good firing position.
In short, with abilities with cooldowns these are the sorts of things that really make more sense when they are directly human-triggered. If the AI is trying to control them, the complexity goes through the roof for us coders, and the frustration is likely to remain high with players anyhow.
A different sort of thing is
contextual abilities with cooldowns. Aka, "he will always fire this when he is able to, if the cooldown is at zero." So in other words, there is no intelligence there on the part of the AI, and you know that. And better yet it's predictable, so your job is to make sure he is in range of something appropriate when his cooldown hits zero. But there again, you don't have enough direct control to really ensure that, so it's kind of frustrating.
Another sort of thing is
periodic abilities with cooldowns. So this is a non-targeted ability to some extent, or at least only somewhat targeted, and it happens every X turns no matter what. Demonaica in Valley 2 is a good example of this. But it's easier with him, because even if he casts status effects on a tile your survivors are not on, he's still blocking you and causing problems. And frankly the point is for your survivors to be out of his range in the first place, as the effects are major and hugely negative. Versus in Skyward you'd want the enemy to BE in the range, so that again gets frustrating because there's no guarantee they would be.
Of course cursing free will is a part of this game, but when it comes to your own units (red and blue I mean), I think that it's a fine line. Having the Japanese wandering gods do something like this (they are intentionally rebellious) would be fine. Having bandit villains do something like this would be fine. Even having hero units do this
specifically when targeting the villain only would be fine. There's enough context there in all cases, and those all make sense.
But for regular units? That seems like too much potential for frustration. And honestly I think there's a big risk of making them "too special" for just regular human units. The human units are only supposed to be so special, if you know what I mean, until they get stacked with bonuses from tokens and so on. So I mean, unique is good, but having special powers beyond what the mytho creatures do is taking it way too far.
A marauder for example could have a "pillage" passive ability that allows him to generate a tiny amound of resources when attacking or destroying a building. For example per attack on a building the marauder gains one sheep (or other random resource). If greek infantry is stronger than norse infantry (with perhaps some ability based on the spartans at Thermopylae - increased stats when adjacent to more than one enemy?), then the norse may find a viable strategy in concentrating more on unit production, sacrificing their units in raids on the greeks to gain resources, while the greeks will need more resource production to support their stronger units.
This is much more along the lines of what I have in mind. This is something that is consistent and non-targeted and so forth. You know that every time a marauder hits a building, there's going to be some theft. So that's predictable and universal and interesting. In fact, this could practically be a bonus for all Norse barracks units, and that would be pretty interesting. But some could have greater theft rates than others.
The difficult part is coming up with things that will keep the overall balance the way it is now; with the Greeks being a bit stronger in an overall sense than the Norse as far as human units go, and bandits still being inherantly stronger than both (as they currently seem to be due to higher stats).
I think that coming up with ideas that affect more than just combat (like the pillaging idea) is to some extent interesting. But there's only so much we can really do with that, certainly.