We added a tortoise to our list earlier this year after 10 years of sitting 
I once had a large saltwater tank and I miss it. A sit with a large saltwater tank with a cat would bring back memories of my cat sitting and staring at the fish till she heard birds chirping and ran to her cat tree to watch them and chirp right back at them. I think another tank is in my future but till then would like a sit with one. Maybe they have different types of fishes than I had. Every fish has a different personality btw.
My preferences are: any pet we haven’t taken care of before. 
It may also be a special breed of cats or dogs.
At the moment we are taking care of some rescue ducks and chickens. The ducks are new to us, but we’ve only just started, so I’m not able to share any interesting stories yet. Except that one hen and one rooster are visually impaired, so we have to pick them up and place them outside the chicken run so they can safely hobble around. That’s a first for us, and we really want to do it well because we simply love seeing happy animals.
Oh, and any pet that enjoys a good massage! I just love giving them, and it’s wonderful to see how relaxed they become. At the same time, it’s a great way to build trust and often makes it easier to teach or reinforce commands, whether they’re walking on or off the leash.