There aren’t a lot of other games that translate well to Arma. I often hear from people that are new to Arma. “What’s the Halo mod”, or “how do I get Halo?”
Operation TREBUCHET is a well-made mod by Article 2 studios. So well-made that a lot of other science fiction mods require it. The mod seems to take place during “the Insurrectionists” uprising area, 2517. Before the covenant war. There are not a lot of covenant assets yet, but, like many things in Arma. It’s “underdevelopment”.
Single-player campaign? Eh, not really. You can find a few missions to do on the steam workshop, where the mod shines is in multiplayer. As long as people play as marines and not dawn the OP Spartan armor. That’s the only issue with the mod is the balance. The only finished enemy is the insurrectionists. But you can find expansion mods to add more units like the covenant and the flood. The quality isn’t there, though.
If you haven’t tried it yet, give it a try. Operation Trebuchet is well worth the price of admission.