Well if you're interested I actually got a universe kind of like that already. The two primary human factions built mechs for different reasons, the first The Steel Houses, originally built their mechs, known as Warlords, to help them win a thousand year long war against a species called the Volkites, and eventually began worshiping the Warlords as gods. As I stated before, just think of the Steel Houses as basically what would happen if you give Feudal Lords giant mechs and tell them 'this is your god now. Go play.' The second, the Galactic Dominion have their own mechs called Colossi, originally built to rebellions and riots but have been modified to combat the Steel House Mechs, due to the fact the only thing at the time that can pretty much kill a Warlord is a another, a shit ton of tanks or nukes.
The two factions aren't good the good guys by a mile. The Galactic Dominion's senate is easily corrupt, expansionists who aren't afraid of using underhanded and shady tactics to get what they want. However they aren't the bad guys either as most of the population of the Galactic Dominion are rather content with high amounts of domestic technology and they're rather accepting to all species who want to join their cause.
The Steel House are xenophobic to the extreme, and when they go to war, they will have no quarrels of committing genocide against alien controlled and human controlled planets alike, literally killing men, women and children by the tens of billions. However the Steel House follow have a code of honor and to some are considered 'modern' day knights in shining armor. They're rather content with just being left to their own accords and often times hire out their services to the Galactic Dominion for a high amount of resources and Standards.
The two factions treat their mechs in very different manner. The Steel Houses use their Warlords as one would use a tank. It's big, its heavy, it packs a lot of firepower and you want to use it as a sledgehammer once the infantry thins out the enemy ranks. The Galactic Dominion treat their Colossi as a bit more disposible as they're mass producing and pumping out pilots like Consoles so there are several million of them within the Dominion. The Colossi primarily are built to be jack of all trades, as they;re built to be agile and quick for surgical strikes, while at the same time they durable and powerful enough to be used as Shock units, but their lack of armor means they're rather vulnerable to anti-tank weaponry such as missiles or AT guns, meaning you would see Colossi in squads so the sight of several Colossi at once might cause an enemy to panic and retreat.
I know that Warhammer is a PC franschise, maybe an MMO? I'm a gamer but I stick to consoles. I have no idea what Phantom Crash is for sure. Give me an idea and I'll let you know if that's what I'm going for. I'm a guy that's into narrative more than systems based RPs. I'm going for themes and story beats rather than stats-heavy stuff.