About Void Sense:

It's not the DC, it's the fact that it seems vague. I mean for instance in our earlier example lets say you want to look into the central room on the 5th floor. You have never been inside this building, nor do you scry on anyone in particular.

Do you perceive it like the Scry spell (an actual sensor is there and your perceiving it like most people do [they'd have to turn around to see what's going on behind them as if they were standing there])? Can the point from which you perceive move? Do you see out to line of sight or only a specific set range (like within 10 feet of where your perceiving)?

Using the example above can the point go through doors, walls, and floors? Can you essentially "explore" the entire complex without having to go in there? Can the point be seen (like a scrying sensor)? Can someone detect it and realize someones looking ( great if your character is a voyeur )?

If your looking into an area like an auditorium, do you first show up in the center of the area, by the entrance, or do you have to specifically state where it pops up (difficult if you've never been inside)?

Now onto other things:

One thing I've noticed about these elemental classes is that most seem to give increased sensory capabilities (things like blind sense, tremor sense, dark vision, and low light vision). The Void Disciple doesn't.

I mean Sense Void would seem to fill that except that for the fact that you go into a coma like state and are helpless. So I'd give them blindsight or dark vision since without some way of perceiving in the dark, Sense Void takes a hit to it's usefulness (I'm assuming spells that give you stuff like dark vision fall under "magical senses" and therefore have to wait a bit before being able to be used reliably).

If you wanted to give it something original, you could take a look in the Expanded Psionics Handbook has a power that lets you sort of "feel" everything within a certain area around you. You could write it up like this:

Sight of the Void/Void Sight (I'm not good with names ):

Your study of nothingness has progressed to your senses. You can feel the void out to such and such range (I'd start at like 5 or 10 feet and go from there). This lets you tell when there is "something" and "nothing" in a particular space. This negates many of the penalties of blindness and darkness. You suffer no reduction in speed, and you can tell if a creature is within a space. You can only tell the shape of objects while using this and cannot perceive fine features.

Think of it like looking at the world if someone drew it with an etch a sketch, outlines and most details can be perceived, but not things like writing and color. However things such as painting on the wall couldn't be perceived, and writing or carving would only be perceived if it was either carved in deep or raised high.

Heck you could have it so that they'd have to "turn off" their sight to be able to use this (inviting the void into their eyes or something like that) and can be used at will.

On Invisible Spell:

The only one of the top of my head would be Sphere of Ultimate Destruction since it requires people to see it to have a chance against it and since it stays for a couple rounds (I'd think that it would have some sort of effect on the environment).