Anguirus: Duh...It's skreoking Anguirus; Godzilla's BFF
Varan: He's came so close to being redone but yet so far. A modern take on him would fit nicely into the Monsterverse or even the Anime.
Baragon & Megalon: Never been my favorites but nowhere close to my least favorites. I think they both have a lot of potential if done right. (Seeing how they are both burrowers)You could even do a story where they have been mortal enemies for eons. Megalon protects Seatopia from Baragon who is a vicious man-eater. The Seatopians see Megalon as their God/Savour and Baragon as their Satan/Adversary. Would work well in Toho or Monsterverse IMO.
Gorosaurus: Because, why not? Surprised he's never reappeared. He's simple enough but could be tweaked a bit to be a bit more formidable. Since all he really is is a huge Theropod, he could really go anywhere but would work extremely well in the Monsterverse. Maybe he was the "King" of Skull Island until Kong's parents trapped him in a cave where he survived for many many years on cave dwelling prey. Maybe an earthquake releases him from his prison? Lots of things you could do with him.
Hedorah: Ah yes, one of my favorites and (IMHO) most UNDERRATED Kaijus. He has only been redone once and they skreoked all over him; making him as pathetic as Gabara. Hedorah modernized and done right could be one of the most powerful, destructive and terrifying Kaijus ever. Hell, the original was one of G's toughest fights in the Showa era. Modernize him and give him back all that was stripped for FW, and you have something truly awesome. Soooo many possibilities and would work in any series. He could even be a different species who evolved in a similar way to Shin but was either of extraterrestrial origin or some creature unknown by science. Then you could have him face off against Shin. Where's my ticket? Lol
Gigan: One of the more popular Kaijus but only appeared once outside of the Showa era. I personally LOVE the FW design and wouldn't mind seeing something similar in a future movie. I've always thought it would be neat to do a Gigan origin story. Maybe about how he was some space dinosaur/avian/whatever the skreonk he was before he was turned into a cyborg by the Nebulans and then show them capture, cyberize (is that even a word? Lol) and raid/conquer other planets using their new weapon. He would also fit nicely in both universes though I don't know what his origin might be if he were in the Monsterverse.
Titanosaurus: would work in a similar way as Gorosaurus. Also not in my favorites but again not least favorites either. I always wanted to see him return. Need more amphibious type Kaijus other than just G. Modernize him, give more power and give him some sort of projectile or breath weapon. I think he could look really awesome modernized in any universe.
And finally (though I'd have chosen a few more not on the list) my 9th choice not listed...
Destoroyah: What's there to say really?! A truly bad a** skreoking Kaiju though overrated as skreok, was still one of G's toughest adversaries of all time. Just imagine a modern Destoroyah. Let that sink in for a minute. Totally. Freaking. Awesome.