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.