So... currently, I'm making progress with Marisa!
I'm feeling some concern that she's not going to synergize well, though, as her current uniques feel a bit scattered. Reading up on Marisa, two aspects to her stood out in my mind -- her ability to learn from her opponents and replicate her attacks, and the nature of the Forest of Magic to deter both humans and youkai from venturing into it, particularly due to the numerous mushrooms there that form the basis of Marisa's magic. There is also Marisa's tendency to steal things from people to consider, as well...
I'm currently using "Stealing the Precious Thing" as her UA, giving her spies a bonus to steal technologies, and she researches techs already researched by her friends faster. It does seem to replicate the "duplicates her enemies attacks" aspect of Marisa nicely.
The Atelier UB currently works kind of the like the Seidi UB from the Sami civilization -- it reveals the presence of Mushroom Patches in surrounding forest, jungle, or marsh tiles. I haven't figured out exactly what the Mushroom Patches will do, but they'll provide both enhanced yields as well as a defensive bonus, in the form of a debuff applied to any enemy units that pass near them. I've got art for a variety of mushroom patches, so considering having three random types that generate different yields -- most likely food, production, and science.
Marisa's UU is a Witch unit... but I'm not entirely sure what to have them do. One idea was to have them venture into foreign territory like the Trickster, and then get a gold/science boost when they returned to friendly territory, to reflect Marisa's tendency to steal things. The main downside is I'm not sure the AI could really use this effectively, and it would likely mean making yet another Scout UU. One alternative that came to mind was to instead focus on Marisa's spellcasting, and make the Witch a Cannon UU (since I'm planning for the Mushroom Patches to upgrade with Chemistry, making it an appealing tech). As a Cannon replacement, they'd likely be able to fire without setting up, making them more mobile, and it would play into the "defensive" nature of the Forest of Magic. I've also considered having them act like Missile Carriers, able to carry a limited number of "Master Spark" spells. Another possibility is that Witches stationed in cities could reveal additional Mushroom Patches...
So which would be the best direction to go with Marisa? A defensive civ that can catch up on technologies fast, using Witches to bombard enemies into submission while protected with their abundant Mushroom Patches? Or should Witches be more mobile, scout units that can steal money and science from other civs -- but may not work as well for the AI? I think the main question at this point is exactly how I want to set up with Witch UUs. The Cannon version works better with the idea of a defensive civ, and I haven't really made one of those yet. On the other hand, the "stealing" Scout version might be more like what people imagine Marisa is like... although less synergistic and being somewhat similar to the Trickster...