A fun prototype is not automatically a game people want more of.
Before spending months on a vertical slice, decide whether this is a commercial project, a portfolio piece, or something you simply want to finish. Those goals lead to different choices. A portfolio game can be short and visually polished. A hobby release can ignore marketability. A commercial game needs a clear audience and a reason for those players to choose it over similar games.
Test that promise cheaply. Write one sentence explaining the game, capture a short clip, and show both to people who never saw the jam. Then let them play without coaching. Pay attention to whether they understand the appeal, replay voluntarily, or ask what happens next. Compliments are weak evidence. “That’s neat” often means they enjoyed five minutes and are done. If nobody wants another session, more content probably will not fix it.
I would push back slightly on making a polished vertical slice too early. @techstream5829 is right that a slice can expose problems, but polish can make a mediocre structure look healthier than it is. First build an ugly second chunk using the same mechanics. If you cannot create fresh situations without adding another major system, the design is already telling you that the scope will balloon. Change the rules so existing pieces combine in more ways, or accept that the finished game should be very short.
Once the idea survives that test, write a fixed release description: platform, approximate play length, control method, number of environments, and what counts as an ending. Treat every new feature as guilty until it proves that it improves the main player experience enough to justify implementation, testing, tutorials, UI, audio, and maintenance. “It would be cool” is not a reason. That phrase has buried plenty of projects.
Keep the jam build untouched, make the full project in a clean branch or new project, and aim for a complete ugly version before polishing individual areas. Title screen, first launch, tutorial, ordinary play, failure, completion, credits, and returning to a saved game should all exist, even if they look terrible. A rough game that can be played from opening to ending gives you something concrete to improve. A gorgeous first level surrounded by placeholders gives you a false sense of progress.
