If I could have anything, my ideal Mac setup would be an ultraportable MacBook, a Mac Mini and a retina class external display I could share between the two.
In some ways this is already possible. If i brought a current generation Mac Mini and a current generation MacBook they would drive one of the 4k panels on the market.
However, they wouldn’t be able to run the display at 60hz, and the Mac Mini would cost over £950 and still be slower than my current Mac.
I’d prefer to use an Apple display if i can (i really like my cinema display and a 4k model at 24” would be amazing!) but i might be willing to flex on that part.
Mainstream Macs capable of 4k 60hz support wont hit the market till 2017 when intel ships CPU’s which are capable.
Till then, I am really happy with my current setup. A portable MacBook1 for writing, light coding, presentations and working on the move and a solid desktop with the wonderful cinema display as a server and workstation.
 The exact MacBook i use might change, as i could do with something that fits on my lap. Will see what happens.