The answer really depends on who you are targeting. I am assuming here that it is primarily SMEs in which case I would probably focus on sales and productivity tools.
There are a number of tools like Pipedrive (Sales CRM) and Trello (List organizer) that I cannot live without. Whatever you develop I think it is key also to focus on plug-ins and integrations with other tools, that can increase stickiness as well as attract users of existing tools.

Whatever you do, ensure it's perfect for the target audience of your paid product.

I've seen people spend thousands on development of a growth marketing tool only to realise the audience that uses it has nothing to do with the paid audience they're trying to attract.

Whatever you do, ensure you're relevant.

If you let me know more about your paid product I'd love to give you some suggestions here. Drop me a PM and I'd be glad to help.