I think the most important feature is the inclusion of the tracking pixel in an email created in Thunderbird.
Another important feature would be the facility to select HubSpot templates and Hubspot snippets in the emails.
That’s probably all I would use, but there maybe other features, such as linking calendars (ie scheduling meetings, etc).
I suppose, it would be worth looking at the functionality of the Outlook Add-in and trying to replicate all that functionality.
I found the image below online for reference.
Kind regards
Stefan