Christmas is nigh, but let's see now what I'd really want for Christmas (and yes, it includes you, but no, it is not all about you). Admittedly, most of what I'll wish for below I'll probably only ever get on my death bed - if at all, but hey, not all the wishes can come true, can they?
One: a built-in wireless headset.
This should include high-quality stereo sound generation and recording facility. It has to be able to connect - and control - various external devices (phones, computers, in-car entertainment systems, etc, etc). The connectivity and control software will need to be upgradeable.
Oh, did I forget to mention where all this should be built-in? Me. Most likely my head. Why faff around with any wires and other silly mechanical bits? Surely it will (soon?) become possible to have a device implanted which will connect directly to the hearing organs. I'm leaving the exact implementation details to those better qualified, but direct neuronal connections can be imagined. These may also be used for the control part. For example, after enabling the system, a double tap of the tip of the index finger could start/stop playback, or maybe initiate a phone call to the last number dialled. And so on. I am sure a convenient control scheme can be worked out in due course.
Two: a built-in basic health monitor and emergency dial system.
By now you've certainly figured out I want it inside me. Again, this is something that sounds quite feasible. if not now, then in a couple of decades. I'd settle for the basics: temperature, blood pressure, oxygen levels, hearth rate. When any of these goes outside predefined limits an emergency is raised. This can be done in a couple of ways: an audible alarm through the built-in audio subsystem (see above), and also by dialling the emergency services using the same system if it is connected to a phone or a computer (and the emergency is justified).
Three: a simple GPS receiver.
I won't even bother any more telling you it should be inside the body. It would interface with the connectivity system from the top of the post. It could help emergency services track you if an alarm is raised by the gadget above. An external SatNav/computer/phone could use its readings to aid navigation, too.
Four: an internal battery.
Of course you'll need one. How else did you think all of the above would be powered? By the lemons? Since I am not overly keen on surgery, and even less on having bits of me closed with a flap, I think we'd either need an extremely long lasting battery, or a rechargeable one. I believe a rechargeable one to be preferable, because there are already nifty ways of recharging it without the need for plugging yourself into sockets. yes, I am talking about inductive charging. It is already available, and it's bound to get immensely improved by the time needed for in-body batteries. It's also easy to imagine an extremely comfortable charging matt for oneself. Of course, it's in your own bed! Just make the whole mattress an inductor and you've made sure that your several (or few) hours of sleep every night will well and truly recharge your batteries. And once everyone has one - you won't even have to worry about hotels or sleep-overs.
Do I expect any downsides to these little (not so) futuristic gadgets? No, not really. We're already using all of them - and using them almost a close to our body as if they were inside them (and some may even have tried some literally inside them - but I'm not going to go there). Would there be enough empty space inside us to have them all? Well, apart from the battery, which may have to be a bit more substantial - but not necessarily, everything else should be as tiny as tiny is. And battery, there's bound to be a convenient space somewhere for it, too. If it's going to be recharged almost every night then it needn't be to big in the first place.
So, all in all, I do not see any reason why these enhancements may not be possible in the (near) future. And would I volunteer to test drive them? Absolutely - once they are determined to be reasonably safe and reliable by, for example, having a few of our furry friends try them out first. But would such a trial amount to cruelty? Well, only if the choice of the music played were really appalling...