Interesting concept for a game! I like the idea of the gameplay mechanics and I quite enjoy the idea of using this style of card battling to romance someone. I think the game has a lot of potential, and I speak about it more towards the end of my video, but I think this concept can be expanded by building a slew of characters to "romance", and then building out how the player convinces them to date/etc them (or continue talking to them; whatever!).
The end goal doesn't necessarily have to be dating them or taking them to prom either, but to build up the overall battling feeling and feeling triumphant, I think it's important not to just throw us at characters to fight; it should feel like you are building to something vs. just doing random battles - imo. The game is only super fun if you are going to get a great reward for doing it.
I might start fleshing the game out like this;
Overall concept - what happens, when, where, why? So this could be "Main character has a week to work up the nerve to ask someone out to prom.. time to rely on the heart of the cards and learn how to socialize! If successful, MC gets to take their dream date to prom!"
1: decide if MC is masculine, or (preferably) make them gender neutral - really solidify the training for card game and how to play it
2: build out characters to romance/date/fight (who are they, what motivates them, what are their personality tropes/archetypes, what do they look like, how do they talk to friends, how do they talk to strangers, how are they known at school?)
3: build out how those characters battles would go; hopefully they would not all be the same (color, symbols, lines of dialogue; make them personal and interesting, make us care about them)
4: build out how you meet and the final sequence with those characters (if the game is over the course of one week, are we choosing who we interact with every day? that's six days + prom to decide dialogue for -- are we only meeting them once on the night of prom? maybe they are having a fight, or situation that we help them solve through card battling? maybe they are just there and we charm them because they don't have a date)
I think this is really the basis of what you need to create a game on a large or small scale. From there you could really decide what more you want to do - unique cards from fighting other characters? A stat system which increases elements of your personality, which helps you secure the person you want to take to prom? Silly moments in the gameplay that happen based on your choices as you go through it in a visual novel style?
The game could be really small in scale, have 3 characters you talk to/fight/ask out, or it could be larger -- whatever the development team has the capacity to do.
I just feel there is a lot of potential here!