Although discourse is rich with excellent approach discussions I would suggest it might be interesting to have a GENERAL goal which could address many of the areas that interest people.
My suggestion is to use the Word search application. It could serve as a basis for newcomers to begin to understand Julia with a problem we all can relate to.
You need to address data management:
the letter structure ( 2d ( square) 3d ( cube))
the sought words which can be static or dynamic
the found words which are discovered ( 3d)
interesting emergent relationships and representation of same.
You need to consider efficiency:
how effective is the manner in which you allow participants to interact. How easy is it to populate the structures needed
how quickly can you find the words, how can it be seen to be more efficient using later releases of Julia or more effective algorithms.
can you find new relationships and if so how?
It would serve as a mechanism for teaching and discovery. It would serve as discussion tool for new approaches in a fun and interesting manner.
So it would end up as a 'sticky" topic where people would contribute their solutions to any of the challenges presented.