Marisa express interest in immortality and eternal youth several times in canon work. In Imperishable Night, she said "Although the whole immortality thing is pretty appealing" in extra story (but not the way by eating hourai liver). In Wild and Horned Hermit first chapter, Marisa show highly interest when Kasen talk about life span extension and hopes Kasen will teach her any life extending and immortal techniques. I think this enough to confirm that Marisa is aiming for immortality or at least life extending but may not by turn to youkai.
She was not only interested, but actually trying to create an elixir of immortality, and the only known problem with it was that it was "too large for consumption" (PCB profile). It wouldn't be strange if she actually ends up being immortal one day or another without noone expecting it (although her "borrowing 'til death" excuses will be no longer valid :V).
The idea of Marisa turn to youkai is not bad idea. I think it very interest to see character who born as normal human and developed herself to become beyond human entity that very fun story.
Well, depends.
FS 25 kinda blocked this possibility, as village humans are explicitly not allowed to become youkai or Reimu will kill them for good.
Unless Marisa counts as "not from the village" (unlike the jinyou in FS), becoming a youkai would be digging her own grave. This might very well be Marisa's reason to search others ways to become immortal or at least having a longer lifespan without becoming a youkai like Alice.
Well, depends.
FS 25 kinda blocked this possibility, as village humans are explicitly not allowed to become youkai or Reimu will kill them for good.
Unless Marisa counts as "not from the village" (unlike the jinyou in FS), becoming a youkai would be digging her own grave. This might very well be Marisa's reason to search others ways to become immortal or at least having a longer lifespan without becoming a youkai like Alice.
I don't think so in this point.
I don't think rule from FS 25 blocked possibility that Marisa turn to youkai.
Reference from reimu conversation
when she killed a human that turns into a youkai she said "In Gensokyo, a village human turning into a youkai is the most grievous sin of all". She use specific word "village human" that strict area of this rule to apply only humans who resident in village but in case of Marisa, she is not "village human" anymore. She cut connection with her family and live in forest of magic for long time if one day she decide to turn herself to youkai-magician this rule not apply to her.
Reimu not mention "any human" just "village human" and I'm sure Reimu not forget the fact that not all humans in Gensokyo are village humans. There are some humans that not count as village humans (not resident of village) : Reimu herself (Hakurei shrine not part of village), Marisa (not resident of village and live in forest of magic), Sakuya (belong to SDM and came from outside world), Sanae (belong to Moriya shrine and came from outside world) and Mokou (live in bamboo forest).