The electrical system is 'plug-n-play' in the sense that you can plug something into a dedicated power line and it will work. For instance, you can piggy back anything off the auxiliary power socket under the seat or off the side / rear light. However, anything piggy backed will be limited to the system's 5 amp rule. ie. whatever it is you have plugged in or piggy backed off cannot draw more than 5 amps. Heated grips may well require more than 5 amps, so they may not work.
But, you can get around this quite easily by putting a relay in place. A relay is really just another switch, except worked automatically by electricity, not by hand. It is how BuMW (and all other manufacturers) enable main lights and starter motors to run; lights and motors which draw well over 5 amps. See assorted threads.
So, one way or another, you can add just about anything.
Now the cost and / or other practicabilities. Some systems, like ABS, are very expensive simply for the parts. Think of it rather like building a brand new bike by buying all the parts over the counter.... £3 for a washer, £2 for a gasket.... You will quickly come to well over the cost of a new fully spec'ed bike. The bike's frames / mounting lugs / forks might be different, which will all add to the cost, too.
You can of course save some money by buying in the parts off a breaker. But, that requires you finding them, ordering them, making sure that the, say, 08 version is the same as the 06 version (or it won't fit) etc. etc. Then add in the cost of the new switch gear, unless you simply want to add third party heated grips, which will come with their own switch. Then think about the new displays perhaps needed for the ABS warning lights and self-diagnostic check systems, and maybe a bit of software update so that all the systems can all 'see' the new ABS. The whole lot is not worth the effort or cost, unless you can do it all yourself and take your time sourcing every cheap part you can find. Even then, for a big job, like ABS or traction control it would still not be worth the effort, time and money.
So, in a word or two:
You can (or could) add or take away just about anything. But it may not fit first time around and / or be very expensive in time or money or require other parts to be fitted too. ABS will definitely fall into this category.
Simple add-ons, like heated grips and lights, often sourced from third party suppliers, can be added reasonably cheaply and easily. In reality, these are little different to adding something like a GPS or a dedicated supply for a heated jacket.
