If you're up for more modding, go for it. Extending your costumes mod to link to the main game, or whatever you feel like doing, could be interesting. As well as would be figuring out how to add items to the main game, so that you could place the end game costume in her home cave. Or anything else you'd like.
But this is if you're up for it, I would not want to force you into something you aren't really willing to do, especially since I have no idea how hard of a task I'd be asking you to do. But if you did, I'd certainly appreciate it, and I'm sure others wouldn't mind as well. Ultimately, it's your decision if you want to put in however much effort it takes, though.
For now I think I'll take your suggestion about simply changing the looks of another costume I don't like...I never considered doing that. The thing that confuses me about that is, though, exactly HOW to do it. The end game costume seems to be all of 4 parts for Naija. Other costumes, including her original form look, contain many more parts than that. So how I go about making the switch complete, I'm a little unclear on. As well as removing things like the various capes/cloaks. I could simply delete the file from the Naija folder, but I'm not sure if that would work or not, I've heard that improperly messing with graphics files results in white blocks, rather than simply nothing loadings.
To be specific, if I use your suggested alternative, I'd like to change the jellyfish costume into looking like the end game. Suggestions for the best way to achieve this? When I followed your advice for turning default Naija into end game, it mostly worked, but due to end game only apparently having 4 parts, I ended up still having some leftover original Naija costume pieces, including but not limited to the shawl thingy she has.