Outside of CBSubs, I have to create a field in CB for the Sponsor level, which is not accessible by the user, so that I can use that field to filter my list.

You don't need to use a field to filter a userlist based off subscription. You can do that directly in the Advanced tab of your userlist which lets you add to the userlist query directly. Examples can be found below.

www.joomlapolis.com/forum/149-usage/239397-cb-list

The proposed implementation is to add fields to CB to allow a userlist to search on instead of needing to have the list pre-filtered using an Advanced filter. We'll either extend CBSubs Plans Image or add a new fieldtype for this functionality.

I suppose there's some clever user actions I could integrate to set that field from CBSubs, but I see other fields in the filter list from CBSubs - why not a subscription level field and an "active/inactive" field that I can use as filters for CB Lists?

Yup, you could toggle a field using CBSubs Fields or CB Auto Actions then filter off that. It'd easily be able to reverse the fields value too when they expire or unsubscribe as well so they wouldn't show anymore. Typically this is just done using a simple checkbox field and toggling the value with 1 for subscribed and 0 for not.

Maybe CBSubs could have its own List capability - a means to create lists direct4ly from CBSubs members so that settings such as showing only current members, filtering by member levels, and other details could be built-in.

We're not going to duplicate code. We'll just extend the userlist functionality we already have.