I've seen a lot of 'found family' stuff lately, which is nice, but honestly? The plots that get me clicking are the ones where their dynamic gets flipped. Like, Martin being the one who needs saving for a change. Maybe he gets injured or sick on an expedition, and Chris has to navigate the terrain and their usual banter while being genuinely scared for his brother. It pushes Chris into a protector role he isn't totally comfortable with, and Martin has to deal with being vulnerable. That tension writes itself.
Also, a fun one I stumbled on was a 'body swap' scenario after some Creature Power mishap. The sheer chaos of Martin's boundless enthusiasm trapped in Chris's meticulous brain, and vice versa, could be hilarious. Imagine Chris trying to explain a complex plan with Martin's rambling energy, while Martin attempts to organize a specimen catalog and slowly loses his mind. The comedy potential is massive, but it'd naturally lead to some sweet moments of mutual understanding, too.
Angst with a happy ending seems to be the preferred flavor for most writers in this corner of the fandom. I'm less into the crossovers, unless it's something grounded like a storm stranding them somewhere remote. Just the two of them, relying on each other in a more extreme way than the show usually allows.