One of the things I liked in Starcraft was that each race had a siege unit which outranged base defences, meaning that no defence was unbreakable. In practice this didn't matter because investing in base defences would see you overwhelmed by your opponent's massed units anyway but I liked the idea.
I've been playing a lot of Supreme Commander 2, and I much prefer its gameplay to that of Starcraft1/2. However some things Starcraft 2 does very well which SupCom 2 doesn't, is leveraging upgrades to make early-game units relevant in the late game, and keeping units distinct.
These and other things have got me thinking about what my ideal RTS game would be like. I'm thinking a lot about the arms race between units and defences, and late-game obsolescence of early game units. Here's a hypothetical RTS game:
At tech level 1 your only units are raider buggies - fragile, fast-moving units. They're useful as scouts and for killing other raiders. However tech 1 laser towers are super effective against light armour and can destroy many times their own cost in buggies, so much so that a tech 1 rush attack is impossible.
Tech 2 unlocks light artillery units which outrange laser towers and make base attacks possible. Their weapons are only useful against stationary targets, this and their light armour makes them easy prey for raider buggies. This is where you use those buggies you built at tech 1 - taking out enemy artillery or their escort buggies.
Tech 3 is for main battle tanks. These will become the basic attack unit. Their weapon is effective against mobile units and structures, while laser towers and raider buggies can barely penetrate their armour. However this is also when gunships - air units that tanks can't even attack - are unlocked. Light laser towers and raider buggies can attack air (though until now this wasn't relevant) and so they remain useful. Heavy laser towers that can penetrate tank armour are now available, but cannot attack air and are still outranged by artillery.
(Should heavy laser towers be buildable at tech 2? That way you can
prepare defences against the tanks before they hit the field, but
because there's no air units yet the light laser tower is briefly
obsolete. Perhaps light laser towers are individually upgradable to
heavy lasers at tech 2, so that even though they're available they're
superfluous before tanks appear and you still need to build the lights.)
Now there's a reasonably variety of units on the field and all the basic unit relationships have been introduced. Past that you can have the fun weird stuff that allows for people to be creative and require tactical responses as much as specific counters.
There's still room for expansion if needed. Cloaked units, for example, could give another tier of rock-paper-scissors relationships. If you make light laser towers and raider buggies the only units that detect cloaked units then you've wrung a fourth tier of usefulness out of them, without making them any better in any other situation. (Though I suspect that it'd be getting strained at that point).