The Long War 2 is great, but the modding community can make anything better. TLW2 is a total overhaul that affects almost all of XCOM 2's systems, which ought to make it hard to mod without causing conflicts. However, a useful Long War 2 compatibility list is being regularly updated on Reddit and there are dozens of mods worth subscribing to via XCOM 2's Steam Workshop.
As we're loading a bunch of criss-crossing mods at the same time, normal caveats apply. There's always a chance that conflicts could cause problems, so save regularly. Having said that, I've been playing with all these mods on alongside The Long War 2 and haven't experienced any problems. If you're looking to supplement your Long War 2 campaign, they should be a good bet. To install, simply follow the link to the Workshop page and click the green 'subscribe' button.
New enemies and maps
A Better ADVENT has already been updated to work with The Long War 2, adding even more alien variants to the enemy pool. Instead of raising the difficulty with super-hard monsters (there are quite enough of those in the game already), the mod focuses on giving the aliens a greater range of loadouts and behaviours. Expect baby Chryssalids, Sectoid soldiers and some funky new shades of ADVENT soldier.
In addition to new enemies, consider subscribing to some map mods to mix up terrain generation and introduce some extra challenge to the campaign. The More Maps Pack adds new layouts to terrain generation. Additional Mission Types is worth a look if you want some extra variations on some of The Long War 2's new scenarios. Avenger Events adds a few dilemmas specifically designed for The Long War 2.
The Long War 2 seems to work happily with mods that add new props to your armoury. Capnpub's Accessories Pack adds some excellent headgear, including berets and night vision goggles, that look completely suited to XCOM 2's aesthetic. If you prefer a futuristic look then Stenchfury's Modular Armor and Modular Helmets adds Mass Effect style gear with plenty of sleek surfaces and tiny lights. The armour mod also adds shoulder pieces such as the impractical but cool over-the-shoulder combat lamp. It's very Aliens. Also, as you can see above, you can give soldiers two sets of night-vision goggles for very dark nights.
These mods are all enhanced by the self-explanatory More Armour Colours mod. I love the simple Uniforms mod too, which simply lets you save outfits and apply them to new soldiers much more quickly. If you enjoy XCOM 2's multinational flavour then subscribe to the International Voices pack for extra Russian and Polish voices. Oh, and you naturally might want the Unlocked Customisation mod, which will low-level soldiers access all customisations as soon as you recruit them.
XCOM 2's user interface is decent, but you can always have more data. Perfect Information displays hit, crit and dodge figures to attacks made by XCOM recruits and aliens in combat, which allows you to laugh at aliens that completely whiff a 90 percent shot. Additional Icons adds a collection of stats above alien health bars, displaying the damage of the enemy's primary weapon, their aim stat, movement range and health. There is also an icon that tells you whether an alien is carrying loot or not. This might feel like cheating, but you can toggle any of the stats off in the mod's .ini file.
There are a few small, frivolous mods that are worth a look. Mission Award Variety changes the awards that appear at the end of a mission—I particularly empathise with the "Hates Stun Lancers The Most" award. On the Avenger base view, Restored Avenger Navigation lets you pan the view with WASD again, rather than flicking between rooms.
Those are the best I've found so far. The Steam Workshop and XCOM 2's mod management interface makes it beautifully easy to flick these on or off in any combination, so experiment away. If you've found some good ones, do share them in the comments.