I do hope that Tesla plans on a 5 door model 3 variant before the model Y. I say "hope" because we saw with the model X how Musk ambition can lead to strategic blunders. The extra year spent getting a fancy model X out the door is time and money that Tesla can never get back.
It is fun to speculate on only the vehicles that could build on the model 3 skateboard, but Tesla has yet to prove that they can do significant parallel development.
The obvious move after 2020 is to build a model 2. A true subcompact which will be completely economically competitive with subcompact ICE. The model 2 probably requires both lower cost and higher density batteries than what is available today.