Don't know of any total conversions, but plenty of good mods. Lots of content packs for additional voices/country flags/soldier customization items/map PCPs/mission names/&c.
Additional Mission Types is excellent. There are some that give you a chance to fort up in a building in a City Center map and hold it for 8-10 turns, which is basically unheard of in vanilla. Some will deploy Resistance reinforcements, and it adds a couple projects to distribute mag weapons and predator armor to the Resistance as a whole, which is a nice touch for immersion.
Lifetime Stats integrates a whole performance history sheet for each soldier to the UI when you have them selected. Sure, the shot's 88%, but the soldier only has a 32% hit rate over thirty missions! Excellent for second-guessing yourself.
Tech Tree adds an in-game tech tree.
Grenade Throwing Tweaks removes grenade tile snapping and reduces grenade range while suppressed.
Show Enemies on Mission Planning adds the Shadow Chamber intel to the soldier selection and equipping screen so that you don't have to undo your work and close it out to double-check if you forget what you're facing.
Grimy's Loot Mod. Man, I love this. It adds locked lootboxes to the drop tables, with a Diablo-style tier progression. You research them to unlock them, which lets you choose a class of item to receive a random drop for, with hard-baked bonuses. These can be any of the standard primary weapons as well as armor, swords, Gremlins, psy-amps, pistols, and grenade launchers. It also has an addon to include the Alien Ruler gear in drop tables, at a much lower drop rate.
So the advantages are obvious. You can get some really cool stuff--I just rolled an Advanced Grenade Launcher that gives +4 range and +2 AoE radius froim a purple box. The author says that the highest-end stuff is equivalent to Colonel perks. There's also disadvantages--the boxes take time to research, and could end up giving you useless crap with bad or self-defeating bonuses.
I like it, more than anything else, for the additional layer it gives to the mid and late-game strategic play. You can risk burning research days to open the boxes (Green are 1 day, purple are 2, and I assume it scales from there), but if you do too many you'll fall behind on standard research, leaving you SOL if the drops aren't worth it. Late game it gives your research team something to do beyond cracking data caches, and provides an additional degree of customization, since a lot of the bonuses are stuff you can't get otherwise, things like Gremlins that give a hacking bonus or additional healing charges, or the aforementioned grenade launcher.
He's also published another addon related to a mod with a couple cut-content PCSes which removes PCSes from the drop tables (and makes them obtainable via skulljacking instead), which helps to trim the fat from the drop tables while also making the black market more important (since you can still buy PCSes there).