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As Hasan mentioned, when starting with something like this, its better to start with a specific niche as it would be easier to market it and reach more audience at first, then you can add more tools in different fields but stay in the same macro niche.
With respect to finding ideas that make it worth it for people to pay, you can focus on building a good tool that, when you use it, you personally benefit from it and you can give people free daily/monthly limited credits to try the tool for free and if they want to use it more they would have to subscribe or buy points.