When should I do the former vs the latter? Any heuristics?
How do I balance improving the existing product vs launching new ones? Any thoughts are appreciated :)
Before someone mentions switch to SaaS, although it would be nice, the marketplace I am currently selling at does not allow SaaS.
I see no problem in extending an already successful product with a set of new features. You can always keep your promise regarding one-time licencing, that does not mean you cannot create a business model on top of that that leverages addons for example.
Personally I feel that spinning up a standalone product involves more risk as a rule of thumb, but again, the answer lies within your domain.
If I had the choice I would probably leverage the "already achieved" success and since you believe that this feature will help your customers, I would give them the feature within my product suite. Also more revenue does not exclusively mean higher price. It might also mean *more* clients.
Hope any of this helps.