A 144Hz monitor would be a huge upgrade, I highly recommend spending your money there. You could also get a mechanical keyboard with Cherry MX switches of your choice and a new mouse such as the Mionix Avior/Naos 7000, but a 144Hz monitor would really take advantage of your system, you'd love it.
By the way, I really like your build. The only change I would suggest is an EVGA Supernova G2 850W instead of the Seasonic. The Seasonic is still a great PSU, but the G2 is even better. It has better ripple suppression, voltage regulation and a hybrid fan so the fan wouldn't even spin when you're gaming. It's also Gold certified and probably cheaper than the Seasonic.
Also, you don't really need to overclock your CPU right off the bat as it won't give you a noticeable performance increase, so I think both of my suggestions are better than a new CPU cooler.
EDIT; I agree with TheMagicalWallaby. The Phanteks Enthoo Pro is a much better choice for a case.