i reckon noelle will join the party, but i think we just won't end up fighting with her. there will be some circumstances written into the plot that make it not happen
like in chapter 2, when she's busy standing on the switch, so you can go fight an ambyu-lance
100% I think she replaces Ralsei
This has been foreshadowed a few different times by now; When Noelle is playing through the Dragon Blazer's Ice Palace with Rudy, she mentions that the Mage who can use FireShock has left the party, and on the Weird Route, mentions that the she COULD use IceShock instead. The Ice Palace could be equivalent to a Holiday House dark world, and in these Fire and Ice mages could be analogues to Ralsei and Noelle.
There's also the scene at the end of Chapter 2 where Susie asks if Kris would take Noelle or Ralsei for the festival, which could easily be foreshadowing for this swap.
And in general, I feel like Ralsei and Noelle are set up for some sort of conflict. Ralsei urges Susie to not get too invested in the dark worlds, while Noelle is desperate for some sort of escape from her normal life. Ralsei is terrified of too many dark fountains starting the Roaring, while Noelle has more reason than ever to want to harness the magic of the dark world now that her father is, presumably, in critical condition. And given Susie's anxiety over how to treat her new girlfriend, I could see her siding with Noelle if the two come into conflict, choosing Noelle to keep in the party instead. And with Ralsei hot off the heels of Flowery coaxing him to be more independent and confrontational, there's room for a plot point of him defecting from the party
And on top of that, with Chapter 5 having the last Shadow Crystal Boss and overall being billed as the last standard adventure, Chapter 6 seems set up to be a lot more experimental and story-driven, and a major switch-up in the party could be part of that.
Plus, I just kinda can't see them adding support for a 4th party member. Not only do I think it would just kinda clutter things up, but as is highlighted in Ch5, Deltarune likes to use it's RPG mechanics as meta-element to the story. I feel like Noelle wanting to join the party and Ralsei knowing there's only support for 3 members could be the catalyst for the argument that gets him replaced with Noelle.
Debating about party composition and mechanics from other RPGs is fun and all, but it seems to me that Deltarune mainly uses it's gameplay as an accessory to its story, not the other way around, and from that point of view, I think Ralsei getting replaced is pretty likely, and could be really really cool.
Good question. I'm assuming it just, like, wouldn't work. Like, it wouldn't be allowed to happen in the first place. I could imagine some gag about this where Ralsei is in the back of the line behind Noelle, a battle starts, and Ralsei just gets left out of the battle scene while Susie asks where he went and complains that he's not doing anything.
And I can, just, really vividly imagine the argument that would happen after.
R: Hey! T-There's no room for me!
S: Whaddya mean? Just stand next to Noelle.
R: I-I can't do that!
S: Why...? What's wrong with Noelle?
R: N-No, it's not that! We just... uh... d-don't have room for another party member!
S: Dude, that's not cool! Noelle can totally join!
R: T-THAT'S NOT WHAT I...
(drama ensues)
I personally don't think noelle will be in the party, the way her spells are calculated are weird, and so overpowered compared to any other spells, unless either of these things happen:
a) Noelle doesn't join/replace (doesn't make sense)
b) Noelle joins but is reworked/nerfed (likely)
c) All enemies are hugely powered up, and the rest of the party is reworked/buffed massively