Many table top rpg sessions have devolved into such discussions much to the game master’s [insert negative emotion].

My thought: Her answer is Soul & getting a New Magical Enhancement.

– Enhancing Kaylin’s existing Magic with Mind won’t help much, as a Spell Thief Rogue her spellcasting capabilities are likely primarily about taking her opponent’s spells and using them against them.
– Enhancing Kaylin’s existing Skills with Hand would normally be considered incredibly powerful, as she’s a Rogue, but it’s the one aspect where she doesn’t really need more advantage. She’s already pretty much the only living person over level 100, and she’s a Spell Thief Rogue, which implies all-rounder to me.
– Enhancing Kaylin’s Stats with Body, while universally beneficial, is likely better for Warrior Builds. A Rogue likely has more versatility with their stats than a generic Warrior Class, and so I feel like Kaylin likely won’t consider this all that beneficial.
– Enhancing Kaylin’s Mobility with Swiftness is something she could benefit her, but she’s a Rogue over level 100, she likely already has a wide range of Mobility Skills that she’s highly proficient in. And that’s without considering the possibility of her just being able to move faster and running into the Speedster Problem, where their speed’s fast enough that short sentences take a subjective ten minutes, if not ten weeks.

A new Magical Enhancement, however, greatly broadens Kaylin’s capabilities, allows her to integrate and test the world’s magic system in ways that will allow her to find possible issues for the divines, gives her something new to learn and work on, and potentially allows her to cover some of her weaknesses or empower her strengths in a way that she currently can’t, depending on what she learns.