I have had an android phone for over 2 years now its running stock 4.2 and just got an android tablet over the holiday running 4.1 and have really enjoyed using both but I think that there are two major parts of the OS that need to be addressed (really should have been already) first is the app data cloud saves and sync so that all the app data, game saves are the same on both tablet and phone.
The second is some way to easily stream video, pics, music or anything else onto your living room tv. Now I know that they have put miracast onto the nexus 4 but there is very little tvs that support it, but I don't really want to buy a new tv to get this and there are no dongle. Also I don't really want screen mirroring, this only seems useful for games. What I want is just to send it to the tv and have it on the tv and still be able to do other things on my phone or tablet with out interrupting whats on the tv. And just a little cheap dongle that I plug into my tv and away I go. Google should make this under the nexus program and sell it the same way they did the nexus 7.
I think that an alternative good way to get this to work would be through the ps3 or xbox that a large number of people have in there living room. This could be a little bit more difficult to implement due to sony / Microsoft.
If google implement nothing else in the next update this would be major and such a benefit.