I waited many years to finally play MechWarrior 5: Mercenaries, and it makes me sad how quickly I've moved on from it. It came out late last year, was fun for a bit but didn't make a lasting impression the way open-world sandbox games of that nature often do. The procedurally-generated missions were just a little too boring, and I wish there were more options for deep mech customization. But now that MW5 has mod support, someone's gone and tried to fix both of those issues.
It's called MW5 Mercs Reloaded (opens in new tab), and though its only in beta and still a long way from completion, the amount of big changes it makes to the base game is impressive.
The biggest adjustment is a fully unlocked mechlab that offers an enormous amount of new ways to tweak and tailor your mech. Instead of just fussing over weapon systems, heat syncs, and armor, Reloaded adds new customization features for engine upgrades, five new types of armor, gyros, engine cooling, sensors, arm and leg enhancement slots, and even deeper structural changes. To compensate for all these new options, mechs have been overhauled to have more available hardpoints, which are what you attach modules like rocket launchers to. For mech theorycrafters, Reloaded sounds like a dream come true.
The mod also makes some significant improvements to combat. Most importantly, over 40 custom assault mechs have been added—though they'll still adhere to timeline so you won't see units in combat years before that model of mech was first created. But combat has also been rebalanced to focus less on annoying units like enemy tanks and aerial vehicles and more on actual mech-on-mech battles, with bigger, more durable enemies to trade blows with.
There are some great new features that should make combat a bit more nuanced, too. Climate and time of day will both affect how quickly your mech cools off after using weapon systems, and heat vision has been overhauled to display accurate thermal profiles for enemy mechs. There's a ton of smaller changes too, like coupling Reloaded with another mod that makes you noticeably limp when your mech loses a leg.
It's a big list, so it's worth heading over to Nexus Mods where you can peruse all the changes yourself and see if it's worth trying out. As creator 'navida1' explains, the mod isn't compatible with other mods that change mechs in anyway, but I think that's to be expected.