Open sourcing T-shirt business niche research and trends tool.

Long story short - started a print on demand T-shirt business. Tried a tool called "Merchinformer" found a good niche there. The trial ended and now I'd like to make a tool that does it better. If there is interest I can post it here as well.

I am looking for ways on how to research new niches, SEO and generally how to drive traffic for free (even if it takes time).

Edit: This is for commercial use I am looking to make money for everyone who helps and myself.