So I hope they put a lot of emphasis on the design tools and design resources. They need to do it at the level Apple, Google (with Holo) and Microsoft has done it (with Metro). Make it extremely easy for developers to make beautiful apps with stock resources.
But before they even begin to do this, I strongly urge them to take another look at Unity, and rethink it a little before they go all-in with it for the next 5-10 years. I still find Unity slow and frustrating (usability wise). Unless the apps will also be made to be touch-ready, forget about the idea of having the UI look tablet-ready. It doesn't help much if only the OS is touch-ready, but all the apps aren't, and if it's only the OS, then it will just annoy the PC users.
I also think they need to further hide some of the complexities of the Linux OS, like all the package stuff and command line stuff. A normal user doesn't even want to see that, and will freak out if he does, because he won't understand what it is, and will just get the perception that Linux is too hard for him.