Something close to this, but still not close enough, has been done. An old game called Lords of Chaos. Many may know it as the game the Gollop brothers made after they released Lazer Squad, and/or the spiritual successor to a small game called Chaos.
In LoC, you created a Wizard and used an XP system to teach him spells. There was a long list to choose from, and almost two-thirds of that list were summoning spells that summoned various critters. These critters were your "units", each with their strenghts and weaknesses. A Giant Bat, for example, was weak and useless except that it could fly and had alot of movement points, so they made great scouts. Pixies were weaker than the bats, but they were one of a handful of invisible critters in the game, so they made good spies. Crocodiles were great for fighting in (the very limited amounts of) water, and Pegasi were decent mounts. Dragons were the ultimate critters, but to summon one you needed to brew a special potion, and the main ingredient for that was very hard to find.
The game was divided into several "worlds". The objective of each world was to be the first wizard to cross through the portal that would eventually appear. The problem was that the portal wouldn't appear right away, and never in the same place, and there were also other wizards around trying to beat you to the portal. Completing a world awarded you XP, so your wizard's stats and spell knowledge would grow as you played on.
The game's only real fault was the lack of maps to play in. The original game only had three (a forest, a cave and a wizard's lair) and the expansion pack added only two more (a series of islands and a tomb). The wizard's lair and the tomb were geared for single-player gaming, so they were tough and quite challenging, while the other maps allowed up to four players to try to wipe each other out.