For mods, i would start with some cool custom islands like Isla Duala (Africa), Lingor (Jungle), and Panthera (based on Hungary, i believe. Mountainous area), and Thirsk (based on Norway i think, but also comes with a winter version). The island Podagorsk is really good as well, basically like an extention of Chernarus and very detailed.
Then there are many unit-mods which add various armies. Depending on where you are from, there should be a nice mod for you out there.
Personally i always play with the Dutch Army units, but for Germans there is the excellent Bundeswehr mod, etc etc
Other than that, there are some AI mods, like ZEUS which help improve the AI, not just accuracy but actually boosting there skill at finding cover, covering eachother, deploying smoke when under fire, if available, etc.
The possibilities are unlimited, really.
And ofcourse, there are the mods that change the entire era of warfare, like Invasion 44, Unsung and Vietnam The Experience, etc.
EDIT:
And soon, (or actually already available) you can also play with Predators, which have several variants like the Elder Predator, Youngling Predator etc, which have different skills. Some will keep ranged, others will close in for the kill with their wristblades etc.
Made by the same guy who made the excellent Undead mod, Charon. That mod is also really good, as it adds both infected and shambler zombies, and even can offer dynamic infection of the island (especially in combination with the Random Civilian Module, which spawns civilians in towns everywhere).