Hello, rohit. Great idea for future development! Time-series support in Pymc3/4 has a lot of things to work on, and adding some good foundation by implementing well the models you listed above would be a very significant step forward.
To start, I suggest you look at a current attempt of building prophet in Pymc3. Luke seems to have done a decent job with it. Also, I would suggest looking at state-space modelling (afaik state-space modelling is the most rigorous basis for flexible time-series), and this for some practical tips.
I am sure mentors will help you out with finer guidance, but make sure to ask any questions here as well in the forum!