I know that Prime 3 is generally liked, but I find it to be very weak in comparison to the rest of the Prime trilogy, its controls, gimmicks, planets (aka zones), upgrades, and getting rid of beam switching all hurt it a lot in my opinion.
I don't believe it's Sylux, it's missing his trademark purple and green colors, and the ship doesn't match properly in shape either. The colors match Dark Samus' Prime 3 design much more, but that's a bit questionable since Dark Samus never has had a ship before and is supposedly dead with the destruction of Phaaze. Who knows, I highly doubt it's Sylux though, the fanbase has always exaggerated his importance and impact in his one game imo. As for the other Hunters, I never really cared about them, their designs and battles were rather poor imo so I never bothered with their backstories, I just knew I had to shoot them down and continue onwards, they were just regular enemies to me. So as you might gather I'm not super thrilled if they go that route again, although I do understand if you do see the appeal of it.
And yeah, I'm more of a fan of the old-school games, although I do hold Prime 1 and 2 in high regard. I definitely prefer Metroid as a sidescroller though, and I can't tell you how relieved I am that they actually have plans for it, we haven't had a sidescroller Metroid in 10 years now so I've been ready to accept that we might never get one again. I guess if they keep on leading the 3D Metroids places I don't like I can always just stick with the sidescrollers, it's easier to accept changes in those imo as gameplay is the deciding factor with those, while in the 3D Metroids immersion is pretty much everything for me.