Integrating a calendar with email is so obvious
That it was done long before Outlook… in Lotus Notes for example.
There's not much of a market for an open source clone. Unlike browsers, which get financed by searches via Google, there's no easy money so the choices are: paid only, or freemium. It's a big piece of software so you're going to need a hefty initial investment until you have something that people might pay for. All the time you'll be competing against entirely free products and the online solutions which are getting better all the time.
But there's nothing to stop Munich council commissioning some open source software development. The KDE platform is now sophisticated enough to allow development of a worthy product.