Type: Bachelor or Master thesis, also possible in combination with a project. This topic gives you a lot of freedom concerning the outcome, but it is also free in the sense that you have to research yourself what is out there already and what can realistically be achieved in six months yourself.
Background info: An example of a recent successful chatbot is Woebot.
Goal: Design and build a chatbot using NLP and Deep Learning. The behavior should be non-trivial.