So my friend and I have continued our progression, and just beat the golem. Onto what I've seen:
Cryogen- Wow. Great start to hardmode, this boss is awesome. I love the music for this boss. My favorite part is when it begins to fall apart and spins in place shooting ice everywhere.
Brimstone Elemental- This boss was pretty fun, but we forgot about it and didn't do it until after we had beaten the mechanicals. Despite this, we had some difficulty with it because the brimstone fire hurts really badly. Pretty fun.
Calamitas- EEK. This boss is really hard. Had a lot of fun, and it took three tries before we finally took it down. Combined the expert accessories from this and Brimstone elemental for a pretty cool accessory. It freaked me out when we beat it and it said "A Calamitas Doppleganger has been defeated!". I'm scared to see the Supreme Calamitas when it comes out if it follows the difficulty trend of this, but excited too.
Leviathan- Ahh, sweet sweet revenge. We didn't know what the siren was at the beginning of our playthrough and killed it. We got one shot lol. Now in hard mode it was great to get our revenge on this boss. Unfortunately, we beat it the first time and can't beat it again. Idk what we got right the first time but oh well. Anyway about the boss, I had fun with it. One time the siren decided to fly away into the sky and never return. I assume this was either an anti cheese (if this is the case then probably magic mirroring around alot) or from not being on the newest version. It actually wasn't despawning, just infinitely flying upward. It didn't fly away when we did the battle normally so I wouldn't worry about this unless its messing alot of people up.
This mod is awesome still! Can't wait to see what new items and bosses we fight soon.