If it has not already been retrofitted I'd install the rear frame catch that keeps the rear frame in place when lifting the bike. Ergons are also a good idea - they might possibly conflict with the Shimano break levers and need to be cut to form a bit. On a 2007 I'd replace the front light with a more decent and modern one (again if it has not already been done). Congrats to the SON btw.! A different saddle might also be worth the money - keep in mind that, coming from the stock saddle, you'll probably also need the Brompton Pentaclip. It is expensive but worth it. Standardvise that would be it regarding the basic upgrades, as always, more is possible.

If you post a picture of your bike we might get more ideas.