Ask HN: Which new tech stack on side project?

I’m a freelance, primarily C# developer. Currently I work on a project with React.js frontend and RESTful C# WEB API. Apart from this recent encounter with React.js my background is primarily 10+ years of backend development in C#. I have started a company before which still runs. For that project I developed everything within the “.Net ecosystem” as we had a tight deadline to market. But now I have an idea where I’m not really under any time-constraint, so I might as well develop my skill along the way, so if the project fails I still end up with a better CV and skills I can leverage as a consultant. Further and this is by no means a deal-breaker – but for what it is worth I would like for it to be able to be fully developed on a Mac.

So which tech stack should I use, that also adds some valuable skills to my CV? The project will primarily be a website and probably apps for iOS and Android, where you can create a user, log in and enter a small amount of data, that then can be shared with people you choose to share them with. Furthermore, these information would in time be available to share with other companies in a “SAAS way”.

At the moment I lean towards a MEAN stack – but is a bit wary on the MongoDB backend. Read some critique arguing about the people behind just ignoring 40 years of database evolvement and thinking they can solve everything better – but apart from that no real experience to sway me either way. Angular also appears to have a steep learning curve. A possibility would of course also be to just build it in React.js and C# WEB API mirroring what I work with at the moment, but I don’t feel like that would really move my skillset any further than what I already do during normal working hours. And somehow I don’t really like the tie to Facebook with React..

I hope the question is not too vague and I know there are no right answers, but I am very interested in hearing thoughts behind possible suggestions.