You have a good list (counting the good suggestions already given), and honestly you may find that 55 liters is too big. There are a couple of perspectives on this:
Option 1: Take 55 liters and pack it slackly.
-You have room to add stuff if you need to.
-You can just jam everything in there if you need to leave somewhere quickly.
-If you have room to, you will probably add stuff even if you don't need it.
-Most packs are designed with a fairly narrow load range, which means if you pack under that range, it won't carry right.
Option 2: Get a smaller pack and some packing aids (basically a variety of zippered mesh bags and compression sacks that allow you to fold and roll things very very small).
-You end up with a tightly packed but tiny bag. You have a much easier time fitting through doors, maneuvering through crowds, stowing it under train seats, etc.
-You're forced to really consider the value of every single thing you pack or add as you go.
-My favorite harness system, suspended mesh, tends to only be available on smaller packs (there are a couple of manufacturers who put it on larger packs though).
-You can't add stuff very easily.
-Every time you pack it you have to take a few moments and do it right. Easy to do on your living room floor, not so easy in a crowded hostel with the manager glaring at you because checkout was half an hour ago.
I tend to prefer the absolute smallest bag that will manage to hold all my stuff, so I go with option 2 every time.