I think he's definitely probably not going to be an antagonist.
One of the first thing he says is "I look forward to creating a new future with you"
then helps make the vessel (until the mysterious person (could be genocide route chara for reasons I won't get into) interrupts it.)
I mean, we've never actually seen or heard of Gaster doing anything bad, so it wouldn't be a strech to say that he might even be a good guy considering he's pretty cooperative. Sure, he caused Spamton and Jevil to exist, but at the same time they were just given information, and they went their own ways. Spamton is barely even an antagonist at points, and kind of tries to work together with Kris, and Jevil, while he is aggressive, doesn't really fight out of any malice or anything, just out of freedom.
He also seems to be the one who delivered the prophecy to Ralsei, so if we consider Ralsei to be in the same boat as spamton and jevil, then there are probably some implications there as well.
I generally agree. I think that Gaster is going to be built-up as some kind of mysterious and menacing force that will look like a possible antagonist, and that it might have already started, and that there may even be religious implications involved, but he actually turns out to be a decent person, but maybe with some questionable interests, beliefs, and motivations. An extremely morally gray character. No hero, but certainly not a villain. He's just doing as he pleases, he has no intent to harm others but doesn't really care about others in any real way, either. That's just my guess. However I would love it if Gaster ended up being a secret hero going against an unknown, lingering threat.
And yeah about that one...it's really hard for me to pinpoint that. On one hand, the manner of speech, the beliefs/mindset being displayed, and the format, are uncannily similar to Chara. Hell, it's even been said that the Japanese translations of it look incredibly similar, if not even identical, to Chara's text in Undertale. Furthermore, it is logical that, if the soul in question is the Player's soul (which it almost certainly is), then well, why WOULDN'T Chara be brought over? Remember, when our soul revived Chara, ripping them out of the abyss, Chara was forever bound to our soul, and because our soul was attached to Frisk, it was a weird three-party scenario where Chara was bound to Frisk as well, kind of like a "spirit", but not a wandering soul so much as a present force, representing the essence of someone that once existed in the world. If the bind between our soul and the revived essence of Chara is strong enough, it's not too much of a stretch to say that Chara would have transferred over to Deltarune when our soul did.
The issues with this theory however are simply that Toby really, REALLY emphasizes that, while there are connections between the two games, Deltarune is its own thing. While he could be exaggerating to avoid spoilers, it would be made into a blatant lie by this theory being correct, as an Undertale character would explicitly have a huge impact on Deltarune's "mystery" plot. Not to mention, depending on interpretation, it would like, technically make the Genocide Route canon to Deltarune, which is just a big no, but I don't think that needs to be the case for the voice to be Chara.
Personally I don't think that it is Chara, but I kind of want it to be. I'm a bit of a nostalgia lover.