The enhanced Excel-specific APIs available for developing Office Add-ins that run with Excel 2016 are a nice start. There are some key missing APIs. One area in need of extending is eventing. For workbooks that contain multiple worksheets, it is critical for the Add-in to be able to determine when a worksheet is activated/deactivated so that the Add-in's UI can adapt accordingly. Similarly, the Office Add-in should be able to hook an event so that when the workbook gets closed/shut down (and maybe Saved), the Add-in can persist in-memory state into the workbook.

In compose app scenario there is no way to manage resources.
1. if you open a meeting in OWA that has resources configured, you cannot access to this resources.
2. if you create a meeing in Outlook and you add "Room Mailboxes" as required or optional attendees, OL resolve those partecipants as Resources and you can no longer manage those elements.

Currently it fires on scroll or changing formatting or cell width or a bunch of other random situations. Additionally, it should probably only fire when the user actually changes something, not when I write something with the API

Data Validation support is a key feature requirement for using Office Apps for OLTP systems.
The main idea is to fill the gap between VSTO features and Office Apps. This will help in easy portability of VSTO based apps to Office apps.