Just a thought... I don't play alot of RTS games, but I had this idea for one and wondered what the opinion of it would be. Good, bad, interesting, un-needed, or already done. Most games go with the concept of build a building, then it starts churning out the units you need, basically from out of nowhere. I.E. build a rifle range, and now you have rifle troops that start popping out of it. My idea basically takes the concept of Build a boot camp...it churns out soldiers. Build a rifle range, send different soldiers through that give them rifle skills, build a paradrop school, give them paradrop skills etc. Have a large number of skills, but give each soldier the ability to know only about 3. This lets you customize the units you have. Its pretty simple, but I think it could work pretty well. Right now its just a rough idea, but I think it could work to be different.

"Battle realms" uses this kind of system, a nice idea, but in my opinion it wasn't developed enough.

Populus had an idea like this, where you have the basic civilians and then you train them to be warriors, preists etc. The difference between it and your idea is that you are still producing units out of nowhere. In populus, they get created from houses with people inside. Personally I like your idea better, because in populus the limited supplies makes it more tedious to play. All in all though, the producing of the units should not play a very large part as most of the strategy imo should be focused around using those units rather than creating them. That being said it could still produce interesting gameplay to advance your units like that.

Does this means my unit has to be "produced" 3 times before I can send it to the battle?

And it can get pretty messed up to differentiate units...

The idea has some potential, but you've gotta be careful not to make "unit creation" the center of the game or people will get bored of doing the same thing over and over without really going anywhere (that is, creating units but not doing anything with them) ((Unless you DO want to make unit creation the center of attention )).

You have a base concept, narrow it down to a working concept ;)

Sounds like a good concept, but it also seems like it'd make gameplay frustrating if you were facing a classic 'rush', and had to go through several different steps to get units that could shoot back with any skill.

It is a very rough concept. But obviously, the units straight from inital creation would be able to fight, and would not be totally useless. However, you would not be required or even expected to put them through thier max number of skills to make them useful. But if you wanted a more specialized attack, (I.E. Fighting, paradroping, engineers who could drop into an enemy base, kill surrounding guards, and then take over enemy structures) then it might take a little time and investment to get to that level of ability. However if you just want to send people out to go kill a squad of tanks coming by, just send your grunts straight from boot camp.

If you limited the extra training grounds to only give the units special abilities (like the paradrop ability) and have the basic unit as combat capable then it could work.

Some ideas:
Special weapon training: The unit now uses a special weapon that is effective against a certain type(s) of unit, but is less effective against others. This makes the unit into a specilaist unit and decreases it's flexability but gains a specialization.
Special ability training: This gives the unit a special ability that can change how it is used slightly (like a paradrop ability that allows it to drop in behind enemy lines). It could also include non combat abilities like inhanced vision (night vision, larger LOS, etc), Carriying capacity (for resources), etc
Upgrades: General enhancement to the unit like speed, line of sight, rate of fire, etc.

to simplify the unit creation process you could have waypoints that you could set to a training building (and the training building could have a waypoint set to another training building, etc) so that the training is automatic. The player would just have to set a waypoint path from one building to another to get the design that they want, so all they have to do (once the waypoint chain is set up) is to produce units at their "Baracks" and the units will get produced according to their production chain without any more effort on the player.

This ouwld take more time (and posably more resources) to make the final unit, but that can be factored in as part of the cost for the upgrades. Also as a unit is waiting to enter a training building it can contribute to the defence of the base anyway.

make sure that the upgrades have drawbacks to balance them.

ex:

The player could make an Rambo unit - (Soldieringx3) high attack , high stealth/speed but same hitpoints as 1 soldier or 3 regular soldiers (Soldieringx1)which would have more combined hit points and can attack 3 multiple targets but easier to spot.

Paratroops can jump from transports (no need to land) but will be spread out the target area and generally have light weapons

snipers have longer range, higher damage, stealth but slow reload/movement.

make some stat upgrades mutually exclusive/applicable only once
ex:

Soldieringx2+Sniperx1 = faster sniper but lower damage/range than a regular sniper since the focus was switched to soldiering

Sniperx2+Soldieringx1 = slightly faster sniper with regular sniper damage/range

Sniperx3 = extremely slow but very long range, high damage

The problem would be figuring out which ability takes higher priority over others. But if there are only a few upgrades/limited slots the combinations could be planned beforehand.

Well one major drawback is time. Building a unit with 3x special skills would take 4 times as long to be ready as just taking the guy straight out of boot camp. (1 for the initial build then 1 more for each of the additional trainings) So idealy you would want to create a lot of grunts, many 1x abilities, some 2x abilities, and only a few 3x abilities for specific plans. Especially since each training will also cost resources.

Certain other units would work the same way, with a base unit being produced, then sending it to get "Upgrades" at various other plants.

I think the idea would be better in a turn-based game.
In a RTS, personally I like to be focused on the action, not on the building.