One of my favourite RTS games is a little game called Impossible Creatures.
This game revolves around being able to take 2 animals from a list of over 50 and combine them into one freak of nature. Then you mass produce your freaks of nature, and command them to destroy the enemy "Laboratory" (which is basically Chitty Chitty Bang Bang as a train).
Now, to produce these creatures, you require two resources. Coal and Electricity. Coal has to be gathered by henchmen (which you start with 3 of but you can build more) whereas Electricity is generated by two buildings, lightning rods (of which you can only have four) and Electrical Generators (which can only be built on geysers). Each creature has a different cost, increasing the base attributes of a creature (e.g. Damage, health, defence, speed, sight radius) increases the coal cost and increasing the special powers of each creature (e.g. Having a Whale head provides Sonar Pulse) or making them more versatile in terms of movement (adding a Whale tail to make it amphibious) increases the electricity cost.
Coal and electricity are also used to construct buildings, from Creature chambers (where you produce your creatures) to Genetic Amplifiers (where you can increase the attributes of any of your different types of creature). All buildings are constructed by henchmen (hence why killing your opponent's henchmen can cripple their economy if you attack early enough and hard enough).
With creatures, there is also a rank system, going from I to V. Rank I creatures are the weakest but can be built from the start. All other ranks must be researched in numerical order (II, III, IV and then V) with each level costing more resources to produce). All creatures that can swim are at least Rank II, and all creatures that can fly are at least Rank III. Rank also determines what buildings you can produce. Here's a rundown of how the ranks affect monsters:
I - Weakest, but will often be produced in large groups. Good scouts due to being smaller and often faster than other creatures if built right.
II - Bread and butter. These guys will still be in semi-large groups but they can hold their own in combat either as base defence creatures or early-game offence creatures.
III - Now we're getting stronger, this is where we get stuff like bears and wolverines. Probably travel in smaller groups than Rank II creatures due to increased cost.
IV - Elite units. Stuff like Tiger-Gorillas and Hippo-crocs belong here. Very nasty in mid-late game. Probably groups of 5-10.
V - The true heavy hitters. These are the Whale-Elephants, the Scorpion-Shark, the Dragonfly-Whale (which is oddly, the best melee unit for damage in the game). These guys are big and hard to take down without large numbers or a Rank V of your own. It's understandable to have one on its own, but often grouped with no more than 4 others.
The ability system is immense. There are some crossover abilities (e.g. Both the Rhino, the Bull, the Hippo, the Ox and the Elephant have the Charge Attack ability), but generally, there are creatures with unique abilities (e.g. The rat is the only creature with Plague).
Here's a few important ones:
Barrier Destroy - Not only are pincers really good combat weapons, they also make a creature immune to "bramble fences" and increase their damage even more against buildings. This makes them base-ruiners wherever they go.
Swimming - A lot of maps have water, and if you're not taking advantage of it, then you'll be at a loss. It gives you more angles to attack from, especially if the enemy base is close to the shoreline.
Horns/Quills - The horned creatures and the porcupine have a very special role. Horned creatures lower the target's defence in melee, and the porcupine can spit spikes at the enemy that do the same from a distance. I'd argue that a big enough Porcupine is one of the best ranged units in the game if your opponent likes to use bid defence monsters like Scorpions, Lobsters and Snapping Turtles.
Herding/Pack Hunter - Two inherent abilities (cannot be taken off a creature as they aren't related to parts). Herding increases defence in groups of 4 or more of the same creature. Pack Hunter increases attack in groups of 3 or more of the same creature.
Flying - Fliers are the best scouts, the fastest creatures (usually) and can bring the pain on an unprepared enemy. But mess them up and they'll be dead very quickly (Anti-Air towers are their bane and can hurt like hell)
Regeneration - Thank god this one is inherent too. No creature heals over time. This is one of two ways to heal a creature (the other being an upgrade for henchmen but that stops them from gathering coal for a little bit).
And the final note: This game was made directly by Microsoft Game Studios in association with Relic Entertainment (the geniuses who brought you Dawn of War).
Yeah, wall of text I know, but this game is good in my opinion.
