This is an idea I had for a game, with major inspiration from Battletech, Lego, and Fallout. Minor inspiration comes from Banjo-Kazooie: Nuts & Bolts, Sound of the Sky/Sora no Woto, Big O, and probably a few other things I forgot about.
So, it’s a mech game. Where you build your own mech. But not like Armored Core or Front Mission, where you buy body parts and weapons and put them together to make a mech. No, this goes into much more detail. Instead of buying arms, legs, weapons, etc., you buy armor plates, servos, and so on. (Hence Lego and Nuts & Bolts.) If designing parts is too much for you, you can buy schematics, or buy them pre-assembled.
Of course, you can try out your mech in a simulation to see how well it works. At first, your simulation only covers movement. You can buy software that contains extra simulations, for a price. (Or just pirate it, if you have the connections. The quality of the software varies, depending on where you get it from. Cracked simulations might be unreliable, honeypots, or just plain inaccurate.) Purchasable simulations include AIs to fight, firing ranges, obstacle courses, and so on.
The setting may seem a bit familiar to some of you. Some time in the future from our perspective, there was a war. A very large war, fought with weapons of immense power. This sent civilization back a few hundred years, but things have recovered since then. Modern technology is inspired by pre-war tech, but is has simpler construction and materials. Thus, mechs are common but lack the power and strength of their inspiration. (Hence Fallout, Sound of the Sky, and Big O.)
In the game, there are two kinds of components you can buy. Modern components are of varying quality, but are common and will work with almost anything. Components usually only work with other components from the same manufacturer, due to different standards. However, you can buy bridges that let you mix brands. Pre-war components are rare and expensive, but incredibly powerful. You can use them with modern components via a bridge, (or just by soldering it on in the case of armor plates) but the added power and stress costs means the resulting mech probably won’t last long.
The player receives job offers via email. What jobs you’re offered depend on your notoriety. These missions are from various interested parties such as corporations, governments, or just anybody with cash. You can also explore the world map, delving into ruins. However, all the low-hanging fruit was picked clean long ago. The only ruins left to explore are either treacherous to navigate, claimed by someone, or well guarded.