If you want to lighten up, I think the first thing to do would be to reduce the weight of your gear. Other than for expedition touring in uninhabited parts of the world, 12-15 kilos should be enough. That is much less than what many carry, but it is possible - just look at ultralight backpackers and mountaineers. If you do this, you will not need front panniers, saving another 2-2.5 kilo on the panniers and rack themselves. It will cost some money, but not nearly as much as a high quality light weight bike. You will discover that riding suddenly becomes much more fun, and that your bike is so much more nimble.
Once you have done all this, there may be a good argument for getting a new bike, if only because your old bike may feel a bit too stiff and rigid (who knows). However, it is unlikely you will be able to save more than 2 kilo's, if at all. After all, you also want it to be reliable. Now that you have reduced your total travel weight, you may be able to fit lighter tyres on your existing bike. These will make a big difference, both because of their weight, and because of their lower rolling resistance. The latter is at least as important as weight.
If, after all this, you still want to spend money on a new bike, I would go for a custom frame with Rohloff hub. The custom frame will fit better, and shave off a little bit of weight (not much). The Rohloff will not save weight, but it is just so much better, you will never want to have anything else again. That at least was my experience. However, I would begin by taking less, and spending some money on lighter kit. You don't want to buy a new bike that is unnecessarily heavy and stiff because you are carrying the kitchen sink.
Willem