PHP HHVM support for better performance
HHVM has been in development for quite some time now. As of December last year, HHVM has supported FastCGI. For those not in the loop, HHVM, or HipHop Virtual Machine, is Facebook's method of handling PHP. It translates PHP into x64 machine code using a JIT compiler. It has almost complete support for everything you can normally do with PHP (you can find its compatibility with popular frameworks here), and it performs absurdly well in benchmarks - see http://hhvm.com/blog/1817/fastercgi-with-hhvm for example. It's fully open-source, and very actively being developed.
I'd love to see cPanel implementing this as it provides tremendous performance gains.