Consider creating a single Opportunity for each subscriber with either the annual value or the current Estimated Average Lifetime Value in the ACV field, whichever is more conservative. You will at least be able to run some analysis, but of course you need to have a clear disclaimer on any reports / presentations you present with this data.
Currently we don't analyze renewal opportunities since most of our marketing spend is on acquisition. But you could also create a new "renewal" Opportunity for each subscription each year.
Not the perfect solution, but at least something.