You seem to be assuming that Australia are the only team that will do any improving over the next 5 years.
Interesting question.
India are about a year away from oblivion as SRT, Dravid and Laxman all retire at roughly the same time.
South Africa are past their peak and will need to start rebuilding soon. Their fast bowling stocks are marvelous. Need a new keeper and few new batsmen. Will not be down for too long.
So that is numbers 2 and 3 in teh world who are on the slide.
England are in very good shape now as they are in their "window" of opportunity. At the moment, probably only need a reliable #6 who can bowl some overs as a bonus. (We will get back to England later)
Australia are in the process of a complete rebuild with players like Ponting, Hussey, Haddin and Harris unlikely to be around for the 2013 Ashes.
Names like Pattinson, Cummins, Hazelwood, Faulkner, Wade, Maddinson, Lynn will be part of the mix of a new look Australian team by that time.
Now the interesting part. In five years time, what improvement will this excellent team have in them?
Strauss 35 yo - will not be around in 5 years time.
Cook 27 yo - will be 32 and still a very good batsman I imagine.
Trott 31 yo - will be 36 and either finished or about to finish in five years time.
Pietersen 31 yo - will be 36 and as above.
Bell 29 yo - will be 34 and entering the last years of his career.
Morgan 25 yo - will be 29, but hasn't established himself yet, so time will tell. Still young enough.
Prior 29 yo - will be 34. Could still be playing, but may be usurped by Jonny Bairstow by that time.
Bresnan 26 yo - will be 31 and still around (if form holds up)
Swann 32 yo - will be 37 and probably finished.
Broad 25 yo - will be 30 and very much around.
Onions 29 - will be 34 and finished.
Anderson 29 - as above
Tremlett 30 - as above
Finn 23 - will be 27-28 and at his peak.
England will need a new opening batsman to partner Cook.
May also need another batsmen or three by then. James Taylor? Hildreth? Or maybe a couple guys from the SA u19's team?
Anderson, Tremlett and Onions will be gone, but Finn and Broad will be at their peak. Who will join them? The Saffer Meaker?
Will Bresnan still be around, or will the Kiwi Benny Stokes replace him by then?
Prior may still be around, but if not, Jonny Bairstow would be logical..or Jos Buttler?
Swanny would be gone I imagine, so who is there to replace him? Scotty Bporthwiock the leggy?
A lot can change in five years as the above illustrates.