Now seems like as good a time as any to list the mods I recommend for second or further playthroughs after you want more difficulty and variety. So without further ado, here I go. The names are the ones they use in the launcher for the game. If you have the game on Steam, you should be able to find them on the workshop.
Quality of life mods
Stop wasting my time -- A from-launch mod that originally removed a ton of long animation delays that were associated with most abilities and actions. I'm not sure how much it still does after the patch that added Zip Mode (which, by the way, you SHOULD be using), but I still use it.
Overwatch All/Others -- Self explanatory. A nice convenience for when you are camping on the maps without timers, or if you use a mod that extends or removes timers.
Instant Avenger Menus -- A serious MVP QOL mod. Removes all those slow camera panning effects and delays when moving between base menus and rooms. Also has the effect of making the geoscape feel like it is performing a lot better.
Mission Time Remaining -- This one is fantastic. Once you've played with it for a while it feels like it was always intended to be part of the UI. Knowing exactly how much time there is left until a scan site will disappear forever is a game-changing feature, but in a good way.
Evac All -- This one is even more optional than Overwatch. I wouldn't say its crucial, but if you like moving all your soldiers into the evac zone before giving the order to evac you might like the convenience.
Quick Reload -- In vanilla XCOM 2, the reload animation for every particular soldier has to finish playing before the next queued ability will run. This fixes that oversight. Absolute 100% required mod for multiple playthroughs unless you love having your time wasted.
Armor Variety -- A purely cosmetic mod that lets you equip your soldiers with the looks of any armor in the game, regardless of what they are actually wearing. This allows you to use NPC-only outfits, too, like the high heels some NPCs are wearing or the suit & tie outfit seen on VIPs or the newscasters during cutscenes. There are alternatives to this one, but I like it best.
Gameplay Mods
Configurable Mission Timers -- Do you hate timers? Well, this is the mod for you. While you can configure it in the inis, the default behavior is to increase mission timers by 4 turns. So if you just want missions to be a little less "rush rush rush" (or simply hate the occasional RNG with map generation where a VIP mission is literally impossible to do in the timer), there's no need to configure, just plug and play.
More Maps Pack/World Expansion Project/Maps By Vozati/etc -- There's a ton of different map mods on the Workshop. If you want more maps, you should install them all. The way maps are structured in the game all map mods are inherently compatible.
Long War Toolbox -- This little mod offers a bunch of minor features like more fine-grained customization of the camera rotation, but the real reason you probably want to install it is that it allows real-time adjustment of the base squad size during the geoscape, accessed through the gameplay options menu. It also changes the UI for squad deployment to support larger squad sizes.
Xylth's Crit Chance Change -- In XCOM 2, hit and crit chance are a unified roll. That is to say, if an alien has a 10% chance to hit you, and a 10% chance to crit you, every hit they get will be a crit, rather than an additional roll to see if they crit or not. This mod reduces crit chance by a scaling amount based on how low your aim is. At 85% aim, there is no penalty. Below that number, it slowly starts to degrade crit chance. I find this more impactful than the EU aim rolls mod, but it's up to you which you prefer.
It's Just A Spark -- An edit of the "It's Just A Scratch" mod to be compatible with Shen's Gift DLC. It's Just a Scratch (and this) restores the Long War wound-related feature where your soldiers are only wounded if the damage taken during a mission starts to harm their base health. In other words, in base XCOM 2 any damage at all is a wound. With this mod, a character with 5 base HP and 5 from armor will not be wounded if they only take 5 damage.
Can't Execute Rulers -- A minor mod, but a nice one if you actually wanted Rulers to be a respectable threat. Repeaters can no longer insta-kill Rulers. That's it.
A Better Advent -- This is a big mod. Does more for your gameplay experience than all the other mods listed so far put together. A Better Advent essentially redesigns almost every Advent enemy in the game, offering multiple variations on every enemy, and plumping out the Advent roster in particular. You'll find advent soldiers with shotguns, early armor pips, or sniper rifles. Sectoids who do not use PSI but prefer to fire their plasma pistols (and do not have melee weakness) will spawn from the first mission. Etc. HUGE difficulty spike for the beginning of the game, and moderate overall, so it's a great mod to both make things fresh again and to counteract all the mods that hand XCOM more power.
A Better Advent: Better Pods -- Increases the size of enemy squads with A Better Advent. That's it. A good addition if you change XCOM's default squad size, or get bored in the base game.
Grimy's Loot Mod -- Here's the other major, HUGE mod you definitely want to at least be aware of. Loot Mod adds a fuckton of mods (i.e, slottable upgrades) for every item type in the game. These are defined by rarity in terms of power and drop chance. It adds lootboxes as a core mechanic to the game, which require research time to open, which does a long way to fix the vanilla game's notorious problem of eventually having nothing to use your research department for. It was inspired by Diablo. Normally I'd consider that a black mark of shame, but this mod is actually really good for making the game feel fresh and fun again. It gives XCOM a lot of increased power, but A Better Advent can offset that. There's a definite joy to slotting upgrades into your gremlins or grenade launchers. More customization is never bad, right? Well, I recommend this mod without reservation. Just make sure you use something like A Better Advent to counterbalance it. Also, pay attention to the next mod. It's important.
Mod Everything -- Lets you put upgrades into any item type. Required for Grimy's Loot Mod, and really I wouldn't use this mod without Grimy's Loot Mod, since that's the only mod that actually adds the items you'd need to slot into anything but weapons.
Grimy's Loot Mod -- Alien Rulers -- Adds Alien Rulers loot (and mods) to the loot table of the Loot Mod. If you think mods for the Rulers weapons is too OP (which is a totally understandable opinion), you may want to skip this mod.
Safer Hacking -- A matter of taste. Do you hate how shitty hacking felt for most of the vanilla game? Well, this removes the burdensome penalties of failing hacks. Instead of making the enemy tougher for rolling the dice wrong, you'll just waste an action and get the smaller penalties (being revealed, attracting enemies, etc). This definitely makes the game easier, but considering how the default playstyle with hacking in Iron-Man was just to never go for risky hacks, this may be a good thing.
That's it for now. What about overhauls? Well, we just recently got our first mostly-complete one of any worth: Spectrum. It's still in a very beta/alpha-y stage, however. If people want to know more about it, I'll write a post about it in particular. It has a LOT of details and stuff to cover, so I didn't want to do that here.