My plans for when the dev tools gets released is to make a simple mini game just to get started. Now this probably won't be finished on the first day but more projects with more people might be finished.
Depending on the capabilities of the tools you can expect me to release the Dab emote, that's right, VoiceOverPete's dab right in the first day. at least is the only thing that I think that can be built/tested/released in one day.
I've found a group of people willing to port over technic mods from minecraft to Hytale. I hope that Hytale will have more a efficient architecture under the hood. So that modding goes easy, and does not strain the preformance too much.