The modder who added seamless co-op to Elden Ring just launched an Armored Core 6 co-op mod for up to 6 players, and it even adds PvP invasions

pcgamer.com:

Extremely cool not-an-April-Fool's-joke news alert: Armored Core 6 is co-op now.

I loved the mech combat in Armored Core 6 when I reviewed it last year, and it was a strong enough singleplayer action game to run away with our best action game of the year award, just like Sekiro: Shadows Die Twice in 2019. It was just a bit disappointing, though, that I couldn't play Armored Core in co-op, the way I'd battled through all of FromSoftware's Dark Souls games and big chunks of Elden Ring. If only there was a modder out there who could make such a thing possible—maybe even a modder who already had experience with turning Elden Ring into a proper front-to-back co-op experience? 

Yes, dreams do come true—after teasing Armored Core 6 co-op back in November, modder LukeYui launched an alpha version of the mod on Sunday, enabling co-op play for up to six total players in a match. The mod also enables classic Souls-style PvP invasions, so while you're cruising through missions with your co-op crew up to two invading ACs may arrive to try to slow you down. 

The invasion mechanic is actually a fantastic fit for Armored Core, since at various points in the game's campaign—extremely minor spoilers here—powerful ACs will suddenly arrive to make a mission much harder than you may have expected it to be. I have to imagine other players will make for even tougher match-ups than those surprise boss fights. If you hate the Souls invasion mechanic, though, it's still possible to play this co-op mod without it by modifying one of the mod files. As detailed by LukeYui on Nexusmods: 

You can disable PvP in the «ac6_coop_settings» file by changing «allow_evil_guest = 1» to «allow_evil_guest = 0». This option only matters for the host of the session.

Some other details from the mod page: 

Installing the mod is as simple as unzipping a file in Armored Core 6's Steam install folder. It adds a new «Squad» menu to the garage, as well as a «Break in Mission» menu option for invasions. To use an