Yoiko's Crossover Hell

[Nekomata-sensei]

Ranma should have known his enemies would eventually come together to plot against him. Surprisingly, the only one who didn't, was Ryoga, partly because he got lost, and partly because he later decided he was more honorable than the rest of them and would only accept defeating Ranma if he did so in a one on one fair combat challenge.
Ranma put up a valiant fight, but facing Happosai, Mousse, Taro, Konatsu's step family, Kumon Ryuu, Mikaido Sanezin, and Kuno, all at the same time, was too much, even for him, despite his defeat of Saffron.
Luckily, Happosai was 'honorable' enough that he wouldn't let them kill Ranma when the battle was over. Kuno was knocked out while they plotted what to do. Happosai revealed that he had a 5cm aging mushroom he'd saved (he couldn't use it on himself because he'd go back before puberty and die to do the modifications he made to his ki paths which need to channel sexual energy in certain ways), and Taro revealed he had some locking ladle water (he'd been hoping to curse Happosai and use it on him).
So when Ranma recovered from the battle, she discovered she was now a 5 year old girl. Without curing herself, her family wouldn't accept her back, denying any relations to them. Ranma couldn't really even live on the streets without a last name, but Ryoga encounters her, hears her story, and takes pity on her, letting her take the name Yoiko Hibiki while she's searching for a cure.
Yoiko first travels to China, with Ryoga, who is looking to stop by Jusenkyo for a cure to his Jusenkyo curse. When they find Herb, and try the unlocking kettle, the find it doesn't work, and Herb declares Ranma's been locked for over 3 months (most of which was spent recovering from her injuries and traveling with Ryoga) and that you have to get unlocked in less than 7 weeks or your cursed form will become your new true form, and you'll become immune to Jusenkyo. Herb suggests Ranma use an ancient Musk artifact that travels to other dimensions. In these other dimensions, energy and magic work differently, although with similarities, while sience remains mostly the same. If Ranma can help a 'destined one' stop a great evil in 7 different dimensions, making things just a little bit better than they would be without her, in each one without revealing she comes from another dimension, growing up in each of these dimensions, she can return home again using the device, and which point, because Ranma will have been through at least one dimension where their dimension's form of magic doesn't work, Ranma will be magically flushed, and thus able to dive in Jusenkyo again, into the spring of drowned man, get the curse locked, stay that way for more than 7 weeks, and go on to search for the aging mushrooms, or simply grow up again, as no time will have passed in Ranma's home dimension while traveling the dimensions, and in each dimension Ranma's time will be re-set, her changing back to the age she entered the dimension traveling string in the first place.
The dimensions Yoiko Hibiki might go to along her quest are: (and possible characters that she could replace, gaining their past memories before she gets inserted at the age of five when some accident occurs to them, the Characters have to bear some resemblance to 5 year old onna-Ranma when they are five, her abilities don't have to end up exactly like them as she grows up however, her own personality traits, and additional memories have some influence, and she'll still be guided to the main characters)
Harry Potter world: Possibly replacing Hermione or Ginny
Pokemon world: Possibly replacing Misty
BLEACH world: Possibly replacing Orihime, Yuzu, or Karin
Naruto world: Possibly replacing Sakura
Smallville world: Possibly replacing Tina Greer (yeah, I know she's a villain who dies before the 3rd season, but so what?, I want Ranma to at least have some special powers in this world, super strength and shapeshifting is fine)
Buffy world: Possibly replacing Willow
Marvel world: Possibly replacing Mary Jane, lacking mutant or other granted ability but becoming mystical skilled ass kicking martial arts spiritualist with mystical powers rivaling Dr. Doom or Iron Fist, and fighting skills rivaling Iron Fist, and teaming up with Spiderman.
Sailor Moon world: Possibly replacing Makoto or Michiru

[Heaven's Deamon]

To be perfectly Honest, it seems too contrived, Like random plot devices were stuck together without too much thought. Having him go to ONE other world with a different system that would allow him to undo the lock at some point might make sense, but traveling to multiple worlds like this doesn't, also, having him replace an existing character comlicates it more sence on each world you give him a "do I stay and live out this person's life or do I go?" and what happens to the person when he does leave? do they "wake up" not knowing what has happened since he "replaced" them? Do they know he was there even if the others around them don't?
Using some of these ideas might make for a decent fic, but all of them at once seem to create too much clutter and even more questions.

[FOG3]

So Quantum Leap, laced with a quest to regain one's old identity. Not necessarily a bad concept, but one that has to be done with a great deal of care to not end up substandard.
Mirrors Multiplied and similar works by the Skysaber/Metroanime team is an example of something of this nature that has done well. They however aren't noted for their skill in using crossovers for no reason.
Some of your suggested worlds seem odd to pointless for this.

[Neko-]

The whole world bit kinda reminds me of Sliders.
Overall I think the whole plotline might be a bit too contrived for this to actually result in a fic that is entertaining and readable, but someone else may have another thought on that.
The 5 year old bit and the growing up would make for boring parts of the fic I suppose. You'd need it to actually show how Ranma makes a difference in the character, but ultimatly in all universes it comes down to one specific moment where Ranma leaves the world in a better shape then it would've been without her.
The 7 weeks and 3 months bit seems farfetched to me. Moreso since I'm not sure how long Herb was looking for the kettle. But if Herb was at Jusenkyo when he got cursed (which is plausable), and he needed to come to Japan to find the kettle (which for Ranma the other way around takes 3 months), Herb would've been locked aswell. So the 7 week/3 month idea seems somewhat contrived to me.
It's an idea, but I think it needs some polishing.

[FOG3]

Reading through your setup again, I'm forced to at least tentatively join the crowd against. You're intending to start him out at age five in each universe and have him stick around long enough to grow up, and then make him go after a TEOTWAWKI threat. Why?
An average age of at least 15 years before egress is going to be needed for that. Given we're talking someone whose both stubborn and has their intelligence mainly invested in physical stuff while being slow enough he can't pick up ice skating basics in a few weeks an average age of at least 20 before egress is more reasonable. This leads to around 70 to 105 years spent on this quest. By the time he would return to his original world it would be more alien to him then the others, and everything about it would be a faded memory. Like Shin he would find that his original world would no longer be a place for him.
The premise has and can be used to great effect, but you need to avoid the traps inherent in this leading to things like annoying fanboyism, getting caught in a universe, and whatnot. Treating it more like Dr. Who in the Key to Time arc is probably a better approach.

[crystlshake]

If you want to keep the multi-world aspect one way to help it would be to change the 'help the destined ones' bit to something more ambiguous that allows you to have a variety of 'quest types' per area. Anouther would be to have his age not be reset after he leaves a place, which would be advantageous to some of the worlds he could be popping into. Perhaps the time in each place is limited by some factor as well. The options he has for curing his curse and age problem are kind of limited as is. You could have it so the World Gate artifact thing the Musk have can absorb magic and remove curses (though you would have to provide a reason for Herb not to use it then). Each world could be a representation of a curse or magical effect on him that he has to 'beat'. Probably have restored function (curse removal) after each successful personal quest. A failed quest could equate to non removal of the magic. The reason for Herb not to use it could be that it would have a bad reaction with his dragon lineage and remove some magical effects it grants him or something. Or if Cologne had the artifact have it once in a lifetime kind of deal win or loose. Perhaps Shampoo already had needed to use it for some reason or there is an additional hidden cost to using it or failing to complete the tasks. Strengthening or adding a curse for example or death. Heck maybee a failure adds that person's life as one of the quest areas.
I have to agree though that the complications with replaceing someone should be avoided for the aforementioned should i stay or should i go thing. One solution along that route is to dump Ranma'a consciousness in with the other person into a time share co-op thing like in carrotglace's Insertion. That would be a quite workable angle if you want to use the 'quests' for personal growth, since you can work on two parties silmotaneously. Though deciding who he gets dumped into should be done with the planned growth of each character in mind and not just random or based on appearance.