Installation of Windows apps to the MicroSD has not been mentioned for the Threshold 2 update that is coming on November 12.
These are the expected features for the Threshold 2 update:
- Integrated Skype messaging, calling and video apps. These Universal Windows Platform apps are the replacement for the formerly integrated "Modern"/Metro-Style Skype app. These same, individual Skype apps are coming to Windows 10 Mobile, too.
- Tab preview in Microsoft Edge browser
- Ability to sync Favorites and Reading List items in Edge
- Improvements to Cortana, including the ability to understand inked notes and the addition of more leisure-time event tracking options, such as movie and event reminders.
- UI tweaks, including "more vibrant" title bars; improved context menus on Start; new icons for devices (like Device Manager and Registry Editor).
- Improvements around device activation for those having issues with Windows 10 automatically activating.
- New mode for managing users' default printers.
There are 2 types of programs: 1) the traditional desktop programs that are basically Win32 programs and 2) Windows Store apps (the univeral apps and the Modern/Metro apps).
The Win32 apps can be installed on any storage drive, including MicroSD drives. However, even if you install them to a drive other than C:, they will still use storage space on C:. That's just the way most programs are designed. So installing Office 2016 on drive D: will still occupy some space on drive C:, but still less than installing the whole thing on drive C:. However, as you use the programs more and more, they will occupy more and more space in your hidden AppData folder located on drive C:.
For the Windows Store apps, they must be installed on drive C:. There are workarounds to get them to install on other drives but they're all flaky and unreliable and requiring major tinkering. Not advised at all. Luckily, the Windows Store apps tend to be tiny in size (because they're basically mobile apps).
At some point, there probably will be support for installation of Windows Store apps to external drives but it's not any time soon (maybe with the Redstone update in 2016).
Same thing for OneDrive. OneDrive for Windows 8.1 supported external drives. With Windows 10, you must use drive C:. That makes it far less useful. If you try to choose an external drive for OneDrive for Windows 10, it will appear to work but will fail soon thereafter. On my friend's HP Stream 13, she only has 32GB onboard. The machine originally had 8.1. Worked great with a 128GB MicroSD card which stored her OneDrive and data files. Upgraded to Windows 10 and now she's stuck with the most basic uses. On all my Windows 10 machines (4 of them), I barely use OneDrive anymore. I do have a Surface Pro 2 with i5/8GB/512GB and for that, I have enough space to have my OneDrive on drive C:. 512GB is completely luxury where I don't even manage storage at all since I have so much space. On my SP2 with i5/8GB/256GB, I do have to be somewhat aware of what I install.
On my Windows 8.1 machines (3 of them), OneDrive works great and is located on external media.
This is why you aim for at least a 256GB computer. That gives you enough room to do whatever you need. You won't have to micromanage storage with a 256GB. Even without using the MicroSD drive for OneDrive or Windows Store apps, it's still really useful for pictures, videos, and other data files.
Office 2016 doesn't take much room however.