Currently, you can add custom fields for existing Activities (Emails, Tasks, Appts, etc) as well as custom Activities. However, the issue is that you cannot currently define custom fields that would span all Activity types such that they are visible in the "Activities" views.

This is a problem for consolidated view "special" filtering at the Activities level.

For example, if I want to define Activities associated with a Team (such that all users who are members of that Team can see them by default), I'd like to be able to define a "Related Team" custom field that would be available on all current entities as well as any future custom entities I might add. Assigning the Owner to be the Team creates issues for specific Activity management. So if the Owner of a Task is the Team, for example, a user with delete capability at the user level won't be able to delete the Task (even though they may have created it). So while I can use the Team as the Owner to create "team views" across Activity types, this creates a limitation on the ability for those users to delete the same activities.

This is just one example of where it would be nice to have a place to define custom fields that span all Activity types and are available for general Activity views.

(Note: This enhancement suggestion is related to another I recently posted on Connect regarding Category/Subcategory for Activities).