I created an account just so that I could give feedback here!
I haven't done a full run through, but so far I really enjoy the added content. I love the Swamp Biome, and I think that I actually prefer it underground as opposed to on the surface. Having more biomes is a fantastic thing, and in my opinion too few mods add them.
On that note, would it be at all feasible to have the Void and Hell coexist in the same world? I was thinking that it may be better to have the Void on the outskirts of the map with Hell in the center. Ie, you'd have the majority of the underworld being the Hell Biome, but to the far left and far right of Hell, you would have a transition to the Void (Maybe between 10 and 15 percent of the map on each side for a total of 20 to 30 percent of the underworld being made up of the Void?). I believe that it would make sense from a gameplay perspective, a compatibility perspective, as well as a lore perspective. It would be absolutely awesome to be fighting the Wall of Flesh and transition between Hell and the Void, as it would signal that the end of the map was coming up and you needed to finish the fight fast. Compatibility is pretty much a no-brainer; if you still have Hell in the world, you also have Hellstone, which means it should be innately compatible with other mods that rely on Hellstone. It also makes sense to have the Void on the borders of the world, as it signals the literal end of the map, similarly to how the Ocean signals the end of the map in Vanilla.
Just my two cents, but I look forward to more updates! Keep up the good work.