Travel Packs
Travel packs are luggage designed to be carried: the front-loading, suitcase-like cousin of a hiking pack. The case for them has got stronger rather than weaker, and the reason is airline hand luggage.
The defining feature is the zip. A travel pack opens like a suitcase so you can reach the middle without unpacking the top, which is the single thing that makes living out of a bag bearable. A top-loading hiking pack does not do this and never will.
Size to the cabin, not to the hold. This is the change worth acting on. Carry-on limits have tightened and budget airlines now charge more for a hold bag than the flight in some cases, so a pack in the 35 to 45 litre range that fits the overhead bin is the sweet spot. It also means you cannot be separated from your things.
Look for: stowable shoulder straps and hip belt, so the pack becomes a duffel for check-in and does not get its harness torn off on a belt; lockable zips that meet in the middle; and a detachable daypack if you want one bag to do two jobs, though these are always a compromise on both.
Wheels or not is the real fork. Wheels are better on pavement and airports, which is most travel; a carried pack is better on cobbles, stairs and anywhere with no pavement at all. Wheeled travel packs that also carry exist and weigh about a kilo more for the privilege.
Unchanged: pack it, weigh it, then take a third out.
Part of the guide to backpacks and daypacks.