Imagine if you could smelt Soul Sand into Phaseglass, a new type of block that blocks water, lava, and light but not you and other entities, replacing signs and fence gates. Think of everything you could do with such a block. A light-dark room. Easy light toggles. Perhaps they can be modified. Use them in alchemy to make an invisibility potion.

I'm just spewing out ideas for a block here, don't mind me if my ideal makes absolutely no sense. Phaseglass. Yay.