To undertake such a design reference design is needed. Without reference design it is virtually impossible or so time consuming that it makes no sense to even start. Luke pointed out some of most difficult parts that include memory design but also all other high speed parts (hdmi, usb etc..) then power section position i.e. position of capacitors, positions of power supply ic, then how to route ever increasing number of lines and so on. Even with well documented producers like TI or Freescale with all documentation and all reference cad designs it is hard.
yyep. usually i now just take a reference design and, as best i can, change as little as possible. on the RK3388 PCB i had to rotate the entire Power Management block - ICs, wires, power planes - by 90 degrees, to shoe-horn it into the available space. it worked. saved a *massive* amount of time, not having to redesign a power layout that is known to work.
and you know what? it worked! the power section worked perfectly, first time. ok, so i shorted out some of the inductors and blew up $200 worth of components, on one of the boards, but hey, omelettes and eggs... :)
l.