Yeah the theme is kind of weird. Some ideas:
- hexagonal tetris (there are similar games, but none I found define a line as a ring around a spiral hexagonal grid)
- maybe an arcade game where the map expands by joining fragments of the world together. Probably too much for 6 days
- some weird gardening game with the plant "cells" being hexagons. Would be hard to fit in the merge theme
https://www.redblobgames.com/grids/hexagons/ is immensely useful.