Well, there's many factors to consider, any of which could make the idea, worthwhile or not:
- The market, a new type of buggy whip might not have much demand anymore [1].
- The investment for an MVP, building a rocket for mars is a great idea but may take millions of dollars and years of time.
- The competition, creating a new Search Engine is a great idea, but the field is dominated by big powerful companies like; Google & Bing...
- The novelty, a new JavaScript framework work web apps must be pretty different to attract much attention these days.
- Your expertise, creating a new baby toy with no knowledge of babies may turn out terribly.
- Your passion, if the idea doesn't excite you, then it's probably not worth dedicating your time to.
Passion is probably the most influential factor in my opinion, because it should keep you motivated when things get tough.
[1] https://www.nytimes.com/2010/01/10/business/10digi.html