I own crmtogether.com and we develop an add-in for outlook that integrates with Sage CRM (and other systems in future we hope). We have built our own VSTO plugin but also have the new 365 support. We need to build and maintain our own plugin due to the limitations of the 365API.
I think you should look at what we have done (EG https://vimeo.com/641696552) and think about what is needed for real world integrations. The latest events that allow yuo check if an attachment is available (thats one example...another is adding in a signature) do not inspire and I cannot see how we could use them.
We need ways to update the UI....365 add-ins are practically hidden behind a menu and users have to be trained to find this. We need better events...like on send (i can show you what we do with the VSTO client)...or be allowed send via the api (and then save to a CRM for example).
Happy to meet and talk to anyone that wants to.
crmtogether.com is my busines site.