As for engines, I think the line-up with the addition of 2.0 TFSI (two different power outputs, 180 hp and 211 hp) is pretty complete for now - 2.0 TDI may be coming this year (170 hp?) and perhaps some different power-ouput versions of current engines as the range evolves. So 1.8 TFSI, 2.0 TFSI, 2.0 TDI, 2.7 TDI, 3.0 TDI and 3.2 will pretty much be the options, although there has been some questioning what the future holds for 1.8 TFSI which is very close to the lesser power-output version of 2.0 TFSI.
S5 V8 4.2 will eventually be replaced by a V6 3.0 TFSI (kompressor), but that is probably a year or two away. Maybe this engine will also become an option on the A5 with less power output, the top of the line 3.2 FSI will probably not be around forever considering how the turbos evolve.
I think the most important future changes will happen in the power transmission area. Current manual and multitronic will continue for the foreseeable future, with some new additions like 2.7 TDI manual coming, but more importantly tiptronic will be replaced - or at least replaced on some models - by a new 7-speed S tronic (dual-clutch automatic).
It is also possible a rear sports differential will be added to some quattro models, so called "vectoring quattro". These power transmission changes may start trickling down to the model line-up as early as late this year or perhaps starting some time next year the latest... but who knows exactly.
Sure, some small changes to accessories and options will continue from model year to model year in May (and mid-model year change in November), but I think the most important changes pre-any-facelift-a-few-years-down-the-line are the engine options and especially the power transmission changes.
And of course the addition of A5 Sportback and A5 Cabriolet, maybe next-yearish?
P.S. CO2 has been reduced for most if not all A5 engines for model year 2009 (cars built on week 22/2008 and later).
