I wonder if someone can create a DEGIRO broker plug-in.

There are some unofficial API clients for DEGIRO's trading platform, e.g.
https://github.com/indiketa/degiro-java-client
https://github.com/drag0s/degiro

Some information for potential plug-in developers: DEGIRO doesn't provide demo accounts. A real account can be created within minutes. According to DEGIRO, 'You can register a free account entirely online within 10 minutes. DEGIRO uses derived bank identification to confirm your identity. This means that there is no complicated paperwork to fill in but you will need to make an initial money transfer to gain access to the platform. Please be aware the minimum of this transfer is £0.01.' (for UK)