Great you are considering this, thanks.
As I suggested before my main concern was grouping all phplist sources in my Analytics reports as “a” source. This is not happening now due to the phplistID currently implemented. To my understanding source should be either “phplist” or ideally a setting value I can tune at least globally. Medium should probably just be “email” and for campaign I suggest you could use a sanitize version of the campaigns slug or alternatively the ID.
This could well be a setting value but obviously if you offer personalising per campaign it will offer much more flexibility. If this second case, I suggest the values should be autocompleted from defaults (like advised) so there is at least no need to remember/change every time.
Another possible option of interest might be allowing [] placeholders there which to me would be the only thing that might make utm_term or utm_content of any use.
utm_term and utm_content are optional parameters in Analytics. No harm if they remain there empty.
Hope is of help. Thanks.