I agree with everything Ben said. I think two packs are a better option with a UL for backpacking and something more sturdy, but not obnoxiously big or heavey for international travel. A good size daybag could do the trick.
I traveled independently around the world for 18 months in 2005-06. Of course, this was before I knew anything about UL backpacking, but I was attempting to carry as little as possible. My trip included urban and wilderness areas and everything in between with travel by planes, trains, automobiles, buses, boats, barges, camels, horses, etc. I used an Eagle Creak Intercontinental Journey Pack for the first 12 months. This was a popular travel pack at the time and combined backpack and suitcase funtions.
Travel packs are not so much in vogue any more and for good reason. Although it had great travel and security functions and features, like clam shell loading, zippers, secret compartments inside, all openings lockable and a security wire running through it so you can lock it to anything, for anytime it was out of your site, like in a hostel. However, the harness system was really bad and carried poorly, especially with much weight. There were times, in between mailing things home, that the weight was upwards of 40 lbs and nearly broke my back.
My itiniary had substantial backpacking built in to the last part, so I knew when I got to New Zealand that I was going to buy an actual backpack, which I did and then used that for the final 6 months. Although not UL, it was much better due to the better harness system and ability to carry well even with more weight.
Now if I were to make a similar journey, I would start by analyzing the trip to see how much is urban, how much in developed versus lesser developed countries, how much wilderness areas, etc and whether I was backpacking and how long or just day hiking. For urban or travel in lesser developed countries, I would choose a pack that was made out of sturdier materials than cuben fiber or the other popular UL fabrics. Again, like Ben said, a good 25-30 L regular pack would serve you well.
For international travel, you really do need good security features for when your pack is out of your site. Imagine your pack strapped to the roof of a jalopy bus bumping along the back roads of whereever. You can absolutely count on prying hands searching through your bag for anything valuable or attractive for the taking. In that scenario it is really great to be able to lock every opening of your bag, but this feature really only exists in the clunky, uncomfortable travel bags. However, if you have a larger day bag that you can have with you inside of the bus, you avoid being separated from your bag. You should still carry a lock and perhaps a cable to lock your bag in your lodging, etc.
If however a significant part of your journey was rugged backpacking for multi-day stints of long duration, you may consider an UL pack set-up or just have UL gear in a regular pack.
FYI - some invaluable references I used for trip planning, cheap air tickets, etc.
Check out the web site and the books by Edward Hasbrouk, The Practical Nomad and Airtreks.com, which is a travel agency in san Francisco that specializes in independent, global travel. Edward did work with them, then took a sabatical to update his books. He may or may not be back with Airtreks now. Either way, look into both of them before you get tickets anywhere else.
Hope this helps.