The pack is currently lacking in aesthetic design capability (though that's obviously not the focus, it could still use something more than the vanilla architecture). A couple things that fit well with the theming here could include ZTones or Quark without changing anything about the progression.

As the modpack is basically finished there won't be any additions of new content mods.

Indeed, I agree that it's lacking in aesthetic content, though my primary concern about adding mods like Chisel or ZTones is that loading time/RAM usage are very high prorities of mine, so if I do add cosmetic mods I would likely add smaller ones. I will have to look around to see if I can find anything interesting.

@al132mc he found some mods and these are them: Nifty, ElementMod, and Plated

specifically NiftyBlocks, ElementMod, and Plated.
There also exists a mod simply titled Nifty that only adds stair and fence equivalents of vanilla blocks.

I've added a few cosmetic mods in 1.1.1. but I will leave this issue open as I would be open to adding more.

Perhaps Chisel to make the antimatter world more... varied?