For touring, as you are no doubt aware, a tank bag is really useful (for a bloody great heavy lock if nothing else) - various types are out there and opinions differ..
A power socket(s) is pretty useful for running heated gear, autocom, GPS. BMW ones cost 3 times as much as none BMW..
As far as I'm aware, there are various taller screens out there, none of which are much good by all accounts..
Plusher seats (Corbin) are favoured by some.
Bar risers and footpeg lowering kits (for the elderly and infirm

)
A decent puncture repair kit...
I've done thousands of miles of 2 up touring with the same kit as you plus a BMW tank bag. Found the Ventura rack (with the sports rack attachment) would take ever increasing amounts of stuff bungeed to it including tent, mini deck chairs, water bottles, baguettes etc....
The tank bag is used for a huge Oxford Monster lock and chain (which is our heaviest single item of kit) maps, Autocom unit, minidisc player (pre MP3) and valuables and bits of tat (wallet, phone, etc.). I unclip the tank bag at fuel/ coffee stops and cart it around leaving the rest of the stuff attached to the bike..
Other mods are mainly performance related - suspension, cat eliminator/ Y piece, exhaust, induct and filter, PC, and dyno time.
From posts on here I believe performance pads tend to warp OE discs more rapidly than OE pads.
There are countless threads on tyres here.
If you go further afield or partake in a track day, take some oil, you'll need to top up at some point.
There is a simple fuel tank mod which involves drilling a hole in the filler neck which provides a small increase in capacity (not done this myself). A bigger mod is to procure a custom larger tank for a none ABS bike.
HID lights.
Stainless fixings (BMW fixings are shite).
Can't think of anything else.
HTH
Ade.