A Better Advent 2
An AI overhaul that could go in the tweaks or increased difficulty sections, but its addition of 60 new enemies makes it feel like a great pick to bulk out XCOM 2's existing content. This is a harder Advent, with a huge variety of new enemies to face, and they will most likely kill you. But what's better than being killed by variety?
More Maps Pack
Increase your pool of maps with this wonderful selection, which was updated and improved a dozen times after its first release. No new art here, just more layout possibilities for your missions, ensuring they don't feel too samey no matter how many aliens you take on.
World Expansion Project: ADVENT
Another map pack that works just fine with the first. This one adds ADVENT checkpoints, house raids, traffic stops and dropship pads as XCOM generates its maps.
Maps By Vozati
Even more combinations! Compatible with the above two mods, Vozati's map pack is about triple the size of the More Maps pack. With all three together, you should be well protected from the danger of randomly generated repetition.
Lucubration's Infantry, Sniper, and Escalation classes
Heard of XCOM 1’s Long War mod? These customized soldier types reprise many of its popular roles. The Escalation classes were designed by our own XCOM 2 expert contributor Beaglerush. They're all a good starting point for experimenting with soldier archetypes outside the defaults.