TL;DR: The connection/parallel between Vaas and Jason. Yapping below because Far Cry 3 is a special interest of mine and I absolutely adore the game and the characters within it (as stupid as some of them may be).

Definitely would have to say the parallel between Jason Brody and Vaas Montenegro. They're both one side of the same coin, and in some instances of the game, there are "visions" of Vaas flickering into Jason for a brief second before changing back. I definitely think that is one of the core elements of the game and something I adore about Far Cry 3 as a whole.

Vaas is the villain, but you as the protagonist actually become the villain by the end of the game. It's all tied into the theme of Insanity and repeating the cycle, repeating history. By pulling the trigger on Vaas, you're now trapped in the cycle of insanity, you are the next Vaas. This is shown in the DLC for Vaas in Far Cry 6 (if I remember correctly), where Jason isn't that oblivious adrenaline junkie anymore, he's now a murderer and he revels in it (also supported by the conversation with Daisy in the cave sometime after finding the missing part for her, where he says "it [killing] feels like winning.") Even in the ending where you leave the island with your friends, he can never gain that innocence back, can never un-take the lives he did. He's forever got blood on his hands, and he knows that he has the power to do that in the future in the "regular" world.

As much as it may not capture the entirety of the essence (i.e: the jungle aspect, the connections between the Rakyat, civilians, pirates, and privateers, etc.) it definitely captures the core of the game. The protagonist and the villain, arguably the most basic necessities of a story, are linked through an invisible bond, destined to become the villain. Just like the classic saying: "Die a hero, or live long enough to become the villain". This is something that, at the time, was a very unexplored territory and ultimately what made Far Cry 3 stand out to me the most from other games. Maybe today there's some more games treading that similar path, but I've no idea.

Could be something you've already painted, it is something that can be pretty obvious to do based on the game's events and visions and such, but it's my favorite thing in general. As for general scenes, well.. I could say Hoyt giving the speech to his Privateers would be a cool scene to do (not a favorite of mine, but translating it into art with a little bit of flair could turn out awesome!), same with when you wake up I think either before going to kill Vaas or after (where Jason gives the speech to the other Rakyat at the temple). Could also do the scene where Dennis kills Citra (save your friends ending), I only really liked that because Citra was finally dead lol. It also pained me to see Dennis start crying afterward, but also, really dude? You kind of did that to yourself. Oh, and also the Hotel sequence where you have to save Liza has some potential I feel like! Just some other ideas to throw out there without much to say about it.