Oooooh, very nice to see a Disventure Camp and Total Drama question!! ^^

Ok, so... I think it's very probable the main villain will be Riya. We already know she will be very important, she'll have a plot with Connor, she'll have a plot regarding her persona and... yeah, she gives huge main villain vibes. In season 2 she was the villain only in the last episodes so it would make sense for her being the main villain in All Stars.
I saw some people thinking Alec would be the main villain... I don't think so, I think he would mostly have the plot with Fiore (the plot I care the most!!!!) and... ironically he could be the "hero" of the situation, maybe he could go against Riya... or maybe they could make an alliance, but then there is still Fiore... It really depends, but I don't see Alec being the main villain.
Grett has her plot regarding her self image and Yul and I don't see her having the role she had in season 1, so I don't see her being the mail villain.
Fiore... eeh... Fiore's situation in DCAS can be described as "Heather in Total Drama Action" or potentially even "Bowie in TDI Reboot season 2". She's getting all the karma back (and even more) and... she could do potentially some plans but that won't be the focus. Also for now we saw a pretty "pathetic" and sad Fiore... exactly like Heather in TDA. I think we'll mostly see Fiore dealing with Alec and trying to get nearer and nearer to him and... AAAHH!! Can't wait to see them make up!!
About other characters... I don't see the possibility of a twist villain. I can potentially see Jake turning into the "TDPI Dave of the situation"... You know, Dave with Sky, how much annoying he was... Aaaaahh, to say it in Italian, "so già che questa trama mi fracasserà altamente i coglioni" (try to translate it if you want XDD)...

Ooooohhh!! About the TDI reboot, I really enjoyed it a lot!! It worked WAY more than season seasons like TDPI and TDAS and even TDROTI and I think this is the most solid cast we ever got, even more solid than gen 1!
I enjoyed both seasons. I'm not gonna spoil too much about season 2 because sadly in Italy the last episodes haven't arrived yet and even if I watched the last episode in English I don't want to spoil something to a specific person that follows me...
Btw, Bowie is my fav character for sure, especially in season 1 of the reboot! He's sooo charismatic and I LOVE how he's technically a villain mostly because he's just... very strategic, but he still has this heart we saw when he interacted with Raj (Rajbow very cute, sadly they showed so few scenes about them in season 2) and Emma (then final 5 happened) ^^
With season 2 I started loving Julia too! She's my bf's second TD fav of ALL time (she surpassed Izzy and my bf LOVES Izzy) and so he is always sooo excited when he talks about her... I really can't wait for him to finish season 2 XDDD
Zee is hilarious and he's my aroace icon, season 2 especially showed his huge aspec vibes (the secrets stuff, you know... how he doesn't understand the concept of flirting...). I don't care that much about Caleb but he's very demisexual and the idea behind him is interesting (the subversion of what Justin and Alejandro have been. Caleb is hot, but a loser that never dated anyone). I don't care about his plot with Priya that much. I don't care about Priya in general. It was evident she was going to win season 1 (even if I hopes soooo much to see Bowie's final) and... eh, some scenes with her and Caleb can be cute but... I prefer other stuff way more.
Wayne and Raj are extremely cute, but I wished they didn't flanderize them in season 2 turning them just into Geoff and Brody. I wished they evidenced more their differences (there are, but they are only showed in specific moments. Mostly about Raj being even more against cheating than Wayne and being less naive), but they are still very funny ^^
Axel x Ripper is a sooo fun ship, seeing them so horny was hilarious XDDD
Episode 3 and 4 of season 2... oh, my... the poem XDDDD
Millie is very autistic-coded, it was interesting seeing how she struggled understanding neurotypical messages but... eeeh, don't care that much about her.
Damien is fine but I don't care that much about him. I wished him and Scary Girl interacted more, it's too funny seeing Damien so scared by her and... I wish there were fanarts where she nonconned him XDD
Scary Girl is funny despite being a character only used for jokes and the authors really didn't care about her.
I enjoyed MK way more in season 2 and MKulia is a very very cute ship (even if I admit thanks to Scepterno now I ship Bowlia, with Julia nonconning or at least molesting Bowie, that's interesting XDDD) ^^
Chase is a huuuge idiot and jerk, but I don't hate him, he's just stupid and I don't understand what Emma saw in him XDD
Don't care much about Emma either.
Nichelle is EXTREMELY boring, she's extremely bland, probably my least fav character of this gen.

If you want to know some specific opinions I have about the season or the cast, ask me! ^^
For now I'll say I really really enjoyed both reboot season, we really needed some classic-style seasons written is a soooo good way! The challenges worked, the plots worked, the STRATEGIES!! I LOVED Bowie's one especially!!
I admit that Disventure Camp creates more hype in me than these seasons because it isn't finished yet and because, despite my love for Bowie, he would be probably just in my Top 10 fav TD characters while Fiore entered my Top 10 fav characters of ALL TIME and she's probably in my Top 5 fav TD/DC characters, I love that demon loli sooooo much XDD