USB Type-C would be a nice addition, perhaps sometime in the future but no plans at the moment. Type-C is still a rare front connection option on even the latest generation motherboards, so for the majority of users it would be redundant.
Other things to consider are replacement IO connectors sold by companies like Silverstone and Lian-Li usually retail with quite a high price tag.
I should be receiving samples shortly for evaluation of a new custom made IO PCB from a different manufacturer with shorter cables. If the samples are OK then I could potentially look into whether they can do Type-C connectors in the future.
No, the vented rear corner piece is for the pre-built systems only.
Yes double height Hi-Fi feet would certainly help to reduce airflow restriction when the case is stood on its side.
According to this airflow testing I carried out 92mm slim fans work best when they are at least 2cm away from a flat surface:
A standard double slot GPU has about 3mm - 4mm gap to the side vents, there is then 3mm panel thickness. So if you can get feet that are around 20mm tall you should have sufficient gap for a 92mm fan to perform well.
I don't think double height feet will be available as an option, Overclockers UK stock Lian-Li feet which they will be using in the configurator for you to select. If you want double height I would advise selecting no feet with the case and then ordering your own separately.
