If someone wants an unit to be add, it just requires balancing and finetuning, even the ones that are hard to do if the mod team wants to do it (at will). If debates have been done before, please refresh the newcomers.
No, if a unit is to be added, first it has to be created graphically and modeled by someone willing to spend hours and hours of their time. That's why, when someone says, "Give me a Dragon Prince unit", the response is not humble. We are in fact extremely lucky to have such brilliant unit makers put so much time and effort into the mod and produce so much of what was needed to make presenting entire races possible, where they wouldn't have been before.
I do agree with the concept of making even the advanced units different but not better in every way; for example Waywatchers were originally the sniper-esque units that were suggested in this thread recently, but the others decided they work better as an upgrade for regular Archers; same with Shadow Warriors. The Waywatchers in fact did have an HP penalty (As would have the Shadow Warriors), so it actually became worth it to still build Elf Archers late in the game rather than pumping out squillions of these so-called rare units.
Another idea I wanted to implement was the idea of giving Phoenix and Black Guard units a movement of just one, but a defence of 7, making them finer defenders than human Pikemen, and also representing the fact that these are defenders who mostly languish in the city and if you want them to accompany your troops on invasions, they'll slow you down (Plus they'll never run away from a fight like units with movement two). Again, I was outvoted and (Unless there's a lack of Iron) you'll never see High Elf Spearmen in the modern age.
With earlier versions of the mod I did include a second "WH-balanced" version, which had changes like Troll-sized creatures being weaker on defence, so that they wouldn't be the only unit built by several races quite late into the game, instead having to be supported by regular troops to defend. Another major change was Wizards being at -1HP (They are only Wizards after all!) so there wasn't a huge reason to build loads of them, as the AI occasionally does. But there were so many patches that needed to be put together I couldn't edit a new version every time, so I stopped. And I think everyone's happy playing the current version, so we'll just have to wait for a Civ 4 mod for true unit balance