Although I'm not much of a fan of real-life car mods in general ( although there are lots of exceptions ofcourse ) the possible inclusion of a comprehensive modifying or customization option in GT5 gets me quite excited.
Bodykits ( including bumpers, sideskirts, etc. ) if included suggest a more specific approach to each type of car than those generic rear-spoilers in GT4.
If there are indeed bodykits ( whatever you think of them ) they could imply widening the car ( bigger wheels, suspension, etc. ) to at least fill the arches visually but would also drastically change the characteristics ofcourse.
Not sure it will actually go that far, but I really hope to get some car-type specific ( real-life or even PD-designed ) parts instead of generic and exchangeable parts ( possibly even different for each tuning-company ).
Coupled to livery editor/paintshop this could be epic indeed, although I would settle for a large selection of rims, different mechanical tuning options and a paintshop to be honest.
All this speculation, in which I take part myself, might lead to some unrealistic expectations but at this stage we still can hope.