To elaborate on:
A real-time strategy game where where you can build units, their weapons, bases, armor, defenses etc. from scratch (modding support would be appropriate) and either make them manned (player-controlled), remote-controlled or AI-controlled.
The last one could be made for everything - self-building bases, anyone?
And you wouldn't be limited in terms of design beyond physics bringing you down, so you could make an autonomous floating aircraft carrier for all I care.
Positioning of your systems and armor shape/angle would be important as well.
And don't forget making your own weapons, Loadout-style, but more elaborate.
It could be a RTS or an FPS depending on your playstyle. No restrictions.
(of course some tactics and designs would be better than others, but that's pretty much how those kinds of games play out in the end)
I figured that I could provide some examples of in-game mechanics (very specific, but should give you an idea what I would dig):
Let's say I want to create a basic mech-like unit. On treads. With two rapid-fire cannons on the sides and a big, slow one on the top. Alright.
Now, if I want it to be a manned vehicle, I'd need to make a cockpit, steering systems and extra support systems. It's not very efficient, is it?
Alternatively, I could strap the cockpit and instead install remote-control systems and cameras instead. That way, the pilot is still in full control, but is not in direct danger.
But if I so desire, I may even want to program a semi-autonomous AI (of middling elaborateness) to get rid of human interaction completely. This does allow me to amass armies with relatively little manpower available, but you are not able to create a perfect AI, and it would also make it easier for the enemy to capture the mechs in question.
I may instead want to go for a different approach - supersoldiers. I can design the shape and features of their armor, including, but not limited to interface that shows heart pulse and body temperature, to targeting systems to camera systems, not unlike Google Glass.
I can also design their weapons - what kind of ammunition they'll fire, where do the bullets come out from, at what rate, and other stuff. I can also design the ammunition itself (to a small extent) and the magazine capacity. As well as any accessories like flashlights and scopes.
If I want to, I can make a three-barreled automatic sniper rifle (minigun-like) with a break action mechanism. Or an assault rifle that uses high-voltage batteries as ammo, discharging electricity at the enemies from a tesla coil-type barrel.
I could even make a floating superfortress complete with hangars for the aforementioned mechs, barracks for soldiers with the armor of my own design and armory for the strange weapons above and its own defense systems.
Or I could design and program robots that build complete bases for me when provided the resources.
Of course that game can and will be outside of computing range for quite some time in its complete form, but a slightly dumbed down (while still functional and fun to play) version should not be out of the question.
I believe there was even a game like this, where you could design your own items, but I can't remember its name now...