So, obviously, huge spoilers for Le Mouvement Final! I'd seen this theory before I watched the musical, and sorda wish I'd gotten to see it without this in mind to see what I would have thought on my own!

At first glance, it seems obvious that the musical confirms Sailor Cosmos as a future form of Sailor Moon, either reincarnated or not. She is played by the former Sailor Moon actress and repeatedly refers to "the mistake I made on that day." However, when she finally meets Sailor Moon in the final battle, after a brief conversation with her, she spends most of her time talking to Chibiusa as they watch the final battle and even after the battle ends, she refrains from going again to speak with Usagi, instead telling Chibiusa about her own battle. If she went back in time for her past self, this makes it seem like instead of Usagi, she might be... Chibiusa!! 

This fits along with Chibiusa's character in other ways as well: she has a habit of going back in time, and she calls Eternal Sailor Moon the "ultimate Sailor Moon" rather than herself, reminiscent of Chibiusa's idolization of her mother. She does also call herself "the future Sailor Moon" but that could be either Usagi or Chibiusa. 

Possibly more significant, though, are her inconsistencies with Usagi. Other than the fact that she spends hardly any time speaking with her, Cosmos, while she does mention all of her friends dying, never makes explicit mention to a lover. While this could just be coincidence, Kakyuu makes reference to her own lover like 3 different times, and we don't even know anything about them, making no mention of the theoretical future Mamoru odd. While this could also be because he wasn't reincarnated... like, come on this is Miracle Romance, there is no way Usagi would be reincarnated without Mamoru. Since Chibiusa's romantic future is kept vague, it makes sense to keep it that way for Cosmos as well.

Anyways, the main problem with this theory is that Cosmos constantly talks about how she made the wrong decision in her past, which seems to clarify that she's Usagi... except she doesn't actually use any personal pronouns! Rather than "I made the wrong decision," it's more of a "The wrong decision was made" kind of statement. Because the omittance of pronouns is common in Japanese and awkward to translate into English without pronouns, though, Miss Dream chose to use the "I" pronoun, as it is what implied in this situation. However, it could be Chibiusa going back to correct the wrong decision that her mother made at that battle. 

Alternatively, Chibi Chibi, in similar fashion to old myu, at one point stops Chibiusa from talking to Sailor Moon, saying something along the lines of "the fate of the galaxy rests on this battle." It could be that in the past, Chibiusa intervened and helped Usagi make the decision to save everyone and continue the battle. So rather than thinking Usagi made the wrong choice, it could be that Cosmos thinks she made the wrong choice as Chibiusa to intervene, and that she should have let Eternal Sailor Moon, or "the ultimate form of Sailor Moon" make her own choice. However, after seeing that Usagi makes the same choice anyways, that's when Cosmos realizes she made the correct choice all those years ago in supporting Usagi.

I've seen this theory from a few different people and I'm still not sure what I think of it, except that I find it really really interesting! Cosmos as Chibiusa is something I never would have thought from just reading the manga, but here I think it is quite viable. Also, this tumblr post has some more thoughts about it, though a lot of it is similar to what I just said!

So, what do y'all think? Is LMF Cosmos Usagi, Chibiusa, or maybe even someone else?

O.O my mind is now blown... I haven't seen this theory before!! It's nice to get some additional evidence now after so much time, too!!! (Yay for new Myu!)

I always wondered why there seemed to be no mention of Mamoru when Cosmos talked about her friends dying, too!! I mean, I suppose by then she might have been hardened to the number of times Mamoru has kicked the bucket but still... Their love is supposed to be a significant source of her power, so it always felt strange to me. (I agree - there's no way he wouldn't also be reincarnated XD.)

I think the Chibiusa argument is pretty strong. You've laid it out really concisely!

But... at the same time, I *want* it to be Usagi instead ^^;;;;. I suppose I could have my cake and eat it too by making the case that Sailor Cosmos is SOOOO far in the future and SOOO much happens until then that her identity as Usagi or Chibiusa would in fact be too hard to tell, because the events between Stars and that distant future would change either of them into an unrecognizable version of themselves. I think that's what the author was trying to get at (and did so effectively) - an ever-mysterious future. So, even with the evidence here for Cosmos being Chibiusa, that overarching feeling is hard to breach.

That being said, I will probably walk away from this thread in the next few days slowly integrating the Cosmos=Chibiusa theory into my headcanons XDDD.

I have avoided posting in this thread mainly cause I haven't watch LMF and therefore I can't say if I agree or not based on the musical itself.

HOWEVER, it is a VERY interesting theory!! And it could have some basis with manga & SMC canon just because of the line that Tuxedo Mask says when Chibiusa first awakens as Sailor Chibimoon (and pardon i don' thave the manga in front of me to quote it) which goes something like "the future Sailor Moon has awakened" or something. IF Chibiusa is supposed to be the future Sailor Moon and Usagi loses her transformation abilities once she's crowned Neo Queen Serenity...it honestly would make more sense for Cosmos to be an eventual version of Chibiusa. I honestly have never thought of that in the least bit tbh especially with varying translations of hte manga being different and such, leaving it very open to interpretation.

If SeaofSerenity ever releases a translation, I'd be curious to see how she (our translator) handles the pronoun situation with Cosmos's dialogue. :O