Disclaimer: As with all of these help articles, and indeed with the mod in general, keep in mind that we are NOT EXPERTS. Take our advice at your own risk - what we tell you might not be the most efficient way to do things, but what we tell you is generally how things work for us.
With that done... Hello everyone! Kerensky287 here. Now, I realize that a LOT of people really enjoy the process of designing and suggesting units. The Unit Suggestions pages are evidence of this. But I also realize that a lot of people might not be familiar with what makes an effective unit... again, uh, a lot of the unit suggestions are evidence of this.
Now, before you get up in arms about how I don't know true genius when I see it, etc etc, I will say that almost all of the suggestions, including rejected ones, WOULD have a place elsewhere - perhaps if there were more factions, or if it was a different game entirely, there would be a niche for them to fill. But when you're making a mod or trying to design anything balanced, there's a difference between a GOOD unit and a GREAT unit.
So, let's say you're starting a Red Alert 3 mod. You aren't content with the factions as they are and think that it'd be worthwhile to add in some new stuff. Now, I'm sure that thousands of ideas spring to mind instantly... but nobody has the time to make all those units. So how do you narrow it down?
1) Look at what's missing. A term we throw around a lot in the design process is what "niche" a unit fills. In case you don't know what we mean, a unit's niche is what it's generally designed to do in combat.
Take the Allied Javelin Trooper, for example - a dude with a rocket launcher capable of shooting planes out of the sky, busting tanks, and when in secondary mode, dropping enemy structures before you can blink. The Javelin Trooper is the Allied anti-air, anti-tank, and anti-structure infantry - that means that for the most part, all those roles are filled for the Allies.
Now, what if someone wanted to add the USA Rocket Trooper from Generals on to the Allies? The Rocket Trooper shoots rockets at tanks, planes and structures like the Javelin, but can be micromanaged to increase their rate of fire using the Laser Sight ability. The Rocket Trooper is anti-air, anti-tank and anti-structure... waaaait a second! If you had a choice between the Javelin and Rocket Trooper, which would you pick? It's irrelevant! They fill all the same roles! The Javelin and Rocket Trooper both fill the same niches and so they just don't work within the same faction.
Now, a slight variation: Japan. Japanese infantry include Warriors, their anti-infantry unit, Tankbusters, their anti-tank/anti building unit, Engineers, infiltrators, a scout, a commando and Rocket Angels (which are a little weird, and kind of work on their own). The first thing that jumps to attention is the lack of a dedicated anti-air infantry.
Now, look at the expansion, Uprising - suddenly, they also have the Archer Maiden, a unit capable of doing plenty of damage to aircraft, while being below average against tanks and infantry. Since you can't reasonably expect to defeat an infantry or tank horde with Archer Maidens, they don't really fit in those niches - Archer Maidens are anti-air all the way. This is an example of a good unit concept - it falls squarely into an important niche and while it can do other things, there are still units that do it far better. Niche overlap in small amounts like this is okay.
(Side note: We got rid of the Archer Maiden because it kinda looked stupid, not because it was a bad unit. Plus, our replacement, the Masamune Drone, pulls double-duty as T1 AA infantry and T1 AA navy because it's amphibious. No point in having them both.)
An important thing to mention as well is that occasionally you have exceptions. Soviet Flak Troopers and Tesla Troopers are both able tank-killers, but the Flak Trooper also hits air. Why, you may ask, would you ever build TTs? Well, TTs are so much better at killing tanks that they're practically in a niche of their own. In theory, having Tesla Troopers available means that you'll stop using Flak Troopers for tank work and use them purely for anti-air. This way, the Soviets aren't lacking in Anti Tank power at any point, but you can still get better AT without ending up with a useless unit. This would be an example of BAD DESIGN if Flak Troopers couldn't attack air - you'd end up with 2 units that do the same thing, but one of them does it better.
2) Look at what works well. You may have read Open Sketchbook's essay on how to not suck at design - in some places, it goes into depth on faction technology, the feel of the factions, and so on. This applies to unit design, both from a gameplay and a visual perspective as well!
Each faction plays a certain way - for example, Japan is full of quick units that can switch between multiple roles at once, allowing an army to change its composition and focus on attacking one unit type on the fly. So, let's say you thought of a Soviet anti-infantry tank that could sprout wings to become a fighter plane - sure, that's something they don't have yet, but transforming just isn't a Soviet thing. It's okay to once in a while add an oddball unit like that, but if you suddenly have 20 Soviet units that transform, it doesn't feel like the Soviets anymore.
Gameplay aside, it's also important to keep technology consistent. Allies use plenty of Chronotech and Cryotech, Soviets use electricity and magnetism frequently, and Japan likes their mecha and psychics. While there's certainly technology overlap in real life, it's best to keep that stuff separate in a game. So even though I'm sure your Chrono-Apocalypse Tank would be neat to see in a game, it's probably best to quietly put that idea away for now.
3) Look at HOW it works. This section is a little odd in that it's different in its restrictions - I'm talking about whether or not it's POSSIBLE to use your idea, and how easy it would be to balance it. This section may also vary based on how hard you're willing to try.
If you want to get your new unit ingame, you'll have to model it, texture it and code it. I'm no expert on the first two, but the Red Alert 3 engine can be pretty restrictive at times. So, let's look at an example idea - a Japanese anti-air tank that can transform into a bomber and back, and it's capable of transporting units in either mode. What's its niche? Air and surface transport, with some anti-ground work on the side. The Sudden already covers surface, but it can also disguise, so its niche is unaffected. Does it fit with Japan? Well, it's a transforming unit, so yeah.
Looks good to go... oh wait! How does it drop the units off if its special ability is to transform? Unfortunately, the most limiting aspect of all in Red Alert 3 is that you can only have ONE special ability per unit, which tends to make transports pretty uninteresting, among other things. So this awesome flying transport is unfortunately out of the question.
There are more subtle restrictions in code, but a lot of them require experience to recognize - luckily, there are usually workarounds if you try hard enough, and if not, it's often possible to just change the unit's functionality a bit to make it work in-engine.
Finally, you need to worry about balance. A well-designed unit is able to fulfill its role effectively unless the opponent uses the right counter. Your main concerns here are how easy it is to fulfill the role effectively compared to how easy it is to counter.
A good example: the Hammer Tank. If you spam them, then you'll kill pretty much any tank you run into, and you can run over a lot of infantry too. But if the enemy uses any air at all, you're done. Bringing along Bullfrogs and a few Sickles will give you a well-rounded force, so it's just up to you to make the right numbers of each unit.
A bad example: the Giga Fortress. In one mode, it attacks everything. In the other, it flies and kills ground units from far outside their range. The price helps to balance it out - you won't have too many, so someone who can destroy what you've got will demolish your momentum - but there isn't really a cost-effective way to destroy one. A few Giga Fortresses in the air with fighter support can and will destroy a base in seconds with no chance of retribution - you can't hit the army with ground units because the Fortresses will destroy you, and attacking with air means you have to take care of the fighter escorts first, which will cost you valuable time.
Another bad example: the Tesla Trooper. As I mentioned before, it works in theory. But the thing is, it costs almost as much as a tank, is only slightly more durable than other infantry, and its combination of short range and terrible speed means that it will almost never be able to fulfill its specified role - enemy tanks can just back away and shoot at it forever.
Unfortunately, balance is often pretty subjective, and it's difficult to get a good sense of whether units are balanced or not without a ton of games by skilled players. Still, sometimes balance issues can be seen from a mile away, and that's the sort of thing you should be looking out for here.
If you follow these 3 general rules, you should have a pretty easy time making good quality units. The rest is up to you... now go forth and MAKE SOMETHING!