Alrighty... next in line is Krystalak.
Krystalak... is a bit of a mixed bag. I don't despise him quite the way I do Obsidius- maybe that's just because Krystalak didn't make things personal, lol. Still, his design has a lot of flaws, IMO, and had the IGN poll included him and been for your top two choices, I probably would've picked Firelion or The Visitor over him.
Still... I liked some of the concept art for Krystalak, and to be honest it irks me that Atari replaced his original, more lizard-like face for the one he has now- which looks like some ridiculous tribal mask or something. I wanted my Krystalak to return to that concept more.

So, here's my take. Like I said, the head here is much more like the concept art than the in-game version. Krystalak is just way too human- it makes him very goofy and cartoonish looking, and I think the toothy lizard head he had before looked a lot more menacing anyway.
Then there's that beard. A lot of people absolutely despise the concept and think he shouldn't have it at all- well, I disagree. I do, however, think it needs to shrink some- it's awfully big, and it's kind of the first thing you see when you look at him, like... "Wait a second... does that giant monster have a BEARD? What the hell?!" So, I made it less prominent, but kept it- I think it adds a sort of devilish charm to him, which is fitting for his character.
The tail was another issue for me- it looked to be made entirely out of crystal, and the tip was just sort of a big puffball of spikes. It looked suspiciously like Space Godzilla's tail, and on top of that it sort of made him look like a poodle when he ran on all fours. I was discussing new tail designs with and he mentioned that a lot of Krystalak's tail attacks would be more effective if his tail had more of a point to it, like a stinger or something. A scorpion tail sounded like too much, but I did give him some elongated crystals on the club to help with causing puncture wounds.
Next, and maybe most important, was balance- Krystalak didn't have any. He had a big head, a big beard, a wide chest, and a humongous array of giant crystals on his back- and then a skinny body underneath. Bipedal or quadrupedal, the poor kaiju always looked like his front half weighed about twice as much as his back half, and I imagine if he ever had a meeting with real-world physics he'd topple over pretty quick.
So, I did some redestributing with the crystals- they aren't anywhere near as long as they were, but now they're more spread out on his body instead of being clustered around his shoulders and upper back.
Finally, and... ironically, probably the most noticeable change I made... is the change in colors. Krystalak was originally white with light purple, which I guess was supposed to give him a sort of quartz-y look. That makes sense, alright- I won't argue that. But it's not exactly the most villainous color scheme in the world, is it? To me, it was just one more factor in making him way too cartoony looking to be in the Godzilla franchise. Not that that ever stopped guys like Jet Jaguar, but still. I went with a dark gray to replace the white, darkened his purple areas, and changed his eyes from yellow to green. I personally think it makes him a rather handsome looking villain.
I think that's pretty much it...

That being said, "Firelion", "Lightning Bug", and "The Visitor" are up next. I know he wasn't considered for GU, but I've also kind of got an itch to draw Bagan... >.>