What does the [h+]^3 in the icon mean?

"H+" is a symbol commonly used to represent transhumanism. This symbol, cubed, seems to indicate a higher level of transhumanism. It might also be a play on the fact that this is the third game in the series.

Let's break down [𝑕+]³.

𝑕+ (the 𝑕 is the unicode mathematical italic small h character, \u+1D455(D835+DC55), a specific math glyph) means Human-plus, the next version of man. In the game that means cybernetically augmented people.

The ³ is likely for Human Revolution being the third game in the series.

Taken together [𝑕+]³ may mean "next-human cubed", as in an exponential advance in humanity as opposed to an iterative advance.

Don't listen to the speculations. It's unhealthy.

H+ or h+ is the symbol for transhumanism (the idea of improving the human condition). The "3" simply stands for Deus Ex 3. The reason why H+ is bracketed is because H+^3 is something completely different and they want to (unsuccessfully, it seems) point out the transhuman context.

Fun fact: The whole Daedalus/Icarus dichotomy is based on literature (JBS Hanlane "Daedalus: Or, Science and the Future", in addition to being that famous greek myth).

CyberSkull also makes a lot of sense with the exponent suggestion, since it fits with the theme.