What problem are you trying to solve? Task management, or employee feedback?
My opinion, and it's worth every penny that I'm charging you for it, is that task management as a field is beaten to death a million times over, and your path to unicorn nirvana will be paved with the bones of the countless anonymous dead startups that walked that path before you. Of course, I would have said the same thing about chat apps before Slack came along. But my point holds... there are a million free task management apps out there, mostly lousy. There's no money in making a free app, and there's no reason to pay for one when there are free ones, except for commercial/enterprise customers - and that market is saturated with many players, big and small.
If you really want to build a task management app, you need a niche - a specific kind of worker in a specific industry (as narrowly defined as possible), who cannot be served by any existing apps. That way, you might make money without too much work.
And avoid adding Calendar, Chat, projects, etc! You know what a good feature that actual businesses might want in their task management tool? Not a calendar - they want it to integrate with the Calendar they already use. Why write your own chat, when customers will just ask for it to work with Slack or Skype or Lync or that smoke signals thing they bought and standardized on in 1997 that only talks with other apps via screen-scraping... you get the picture.
More to the point here, though... what is your problem that you want to solve? Are you scratching your own itch for a task management tool after finding the whole field lacking? Or do you just want a startup, and don't have a specific problem, so you looked for something you might do? Not judging here, just making sure you know what problem you're actually trying to solve. Because you're solving both your problem and your customer's problem, and if they're not one and the same, you should really look for a customer problem that solves your problem.