Thank you for taking the time to give extensive feedback!
I'm open to eventually "upgrading" a side character to main heroine if they prove to be particularly popular, but (almost) any side character is likely to have some fans. Making a complete game is the goal rather than trying to keep adding to it endlessly, so 12 full romance routes for every character would be unrealistic. After the game is complete I might go the DLC route for the most popular side character based on a poll or something, but that's still a far off future.