The Microsoft Store has been open since the launch of Windows 8, so I'm surprised it hasn't already been added. Then again, I can't think of many people who actually opt to use it over Steam.
https://www.microsoft.com/en-us/store/b/home

Discord announced their storefront a while ago, but they only recently opened it. They seem to be doing a Twitch Prime sort of deal with Discord Nitro where users get a ton of free games each month. If the Discord store isn't already available to you, then you can get access by redeeming a Discord game key. Here's a free Discord game key giveaway for Minion Masters: https://gleam.io/vCOMB/discord-minion-masters-key

Also, could Humble Bundle be added as an Android digital format as well? Thanks.

Microsoft Store and Discord Store have been added as PC options and Humble Bundle has been added as an Android option!

Yet another PC storefront has been opened, Kartridge. Kartridge is a new digital PC storefront from Kongregate. They're apparently heavily curating the site so that no simple asset flips or objectively low quality games enter the store. Judging by a few giveaways and a general peek at the store, it does seem to show great promise in the "no bad games" department.
https://www.kartridge.com/

Thanks in advance.

Kartridge has been added!