I think something that might be of value to the HubSpot community.
Number of marketing emails opened is greater than X within the past Y days.
A filter which segments contacts based on the number of marketing emails opened within a specific time period would massively help in segregating engaged and unengaged contacts.
Eg. Number of marketing emails opened is greater than 4 within the past 30 days.
Another filter which might help is - Number of marketing emails clicked is greater than or equal to 3 within the past 30 days.
This will give users a chance to treat people who actively (and recently) engaged with your emails a little differently.
Let me know of other ways we can make this filter more useful.
Thanks.