Whilst walking around the past few days, I've been mentally simulating having a backpack with varying amounts of features that are implemented in various ways. I think I've made my simple and inescapable choice: pogo wheel. 

The enter-bag-without-dismount feature seems to add quite a bit of complexity for a situation that relatively happens for a small percentage of time using this imaginary backpack. 

By far the biggest net benefit would be to direct the gravitational force of the backpack straight into the floor, without the literal middle man. The suitcase method has usability challenges, mainly caused by sharp changing elevation requirements (curbs, steps), the payloads would be subject to additional impacts, increasing the risk of damage, and it is unlikely that lifting the entire compartment when switching between wheeled and suspension modes is a simple engineering challenge. 

Compared to this, the pogo-stick wheel is lighter (requiring less power to extend and retract, as well as a fixed weight) and smaller, reducing the likelihood of my feet colliding with anything when moving.

For true manoeuvrability, a 2 directional (normally called an omnidirectional) wheel will be required. To cut down on noise and to ensure compatibility with #TEOSS [gd0037], a wheel with a diameter larger than 100mm is probably a good idea. In research, it seems that wheels that size and up would be needed anyway to support the weight of the bag. I didn't look into it much, but it seems that the swivelling trolley wheels, diameter being equal, is only slightly cheaper. Since these wheels can't roll in different directions instantaneously and the chance of a foot collision is higher, I've decided to skip them.

Currently, I think the features in order of importance is:

1. Can safely carry #Teti [gd0022].
2. Retractable pogo-stick wheel that is safe to use with TEOSS.
3. Soft, rolling straps.
4. Hoverglide suspension.

Wheels I found yesterday:

First thing I saw was this really expensive wheel with a really expensive shipping cost. I think it's for a really expensive mobility scooter. The description says it's 3.7kg so that's out.

This wheel can take 30kg of force and weighs 360g or something, but it also costs more than any backpack I've ever bought. I knew this backpack was going to be much more expensive than whatever I could buy off Amazon/AliExpress, but...

This one is 320g and can do 20kg, but I probably shouldn't try this wheel is when going at elevated speeds. Then again, I just remembered that some kick scooter wheels have plastic rims, so perhaps it's fine:

Now if I'm spending the big bucks, might as well go with the platinum skin. I've got no idea of the durability of this wheel or if said wheel would actually damage the ground, but metal wheels have less friction and this application calls for such low friction wheels. This wheel looks beautiful and I'm currently using this design for all mental conceptural imagery, but apparently it can only carry a measly 5kg. Why? It's also 539g (though most other listings state 250g). I once found a larger 127mm wheel that was essentially £135 considering VAT and could allegedly carry 20kg, but I have even less faith that aluminium would last.

Then there's this 740g wheel that can carry 40kg:

I do wonder if the alternating wheels for all but the "platinum skin" one would cause any usability of safetly issues.