The next version of vSphere will get a HTML5 based Web Client, this will not only replace the C# client but will also replace the current Flash based Web Client. Although both web clients will coexist for some time to give (3rd party) plugins time to move from the Flash based Web Client to the new HTML5 based Web Client.
VMware states that the HTML5 Web Client will bring a great user experience. Currently you can already try the new HTML5 "look and feel" when you run ESXi 6.0 Update 2, this includes the Embedded Host Client which started of as a VMWare Lab Fling but made it into the Update 2 release.
And if you want to take it even a step further in your lab (VMware Lab Flings should not be used in a production environment), you can go ahead and get the vSphere HTML5 Web Client Fling!
At this time, it is not sure if the GA version of the HTML5 Web Client is going to look the same as the Fling does at the moment. But for the moment I really like the clean and basic look (called Clarity) of the Fling.
One other important thing to mention, VMware will try to stay on the same support model (supporting the one it’s released with, and one version back for upgrade transitioning) for the new HTML5 Web Client. Due to the amount of changes to the backend API it is not sure if they will be able to make this actually happen.