Personally I liked to go for self made up completionist stuff like buying all mini game items, all items from shooting star shop, all items from clue scrolls, crystal chest and burnt chest, all unique drops etc...
Could also mention that the pets are kind of in that directory too but they're all so heavily based on RNG. This is coming from ironman perspective because I'm not a fan of buyable boss pets. (Godwars)
In my personal opinion I think the goal for 2019 could be to release achievements (easy, medium, hard, elite) and ultimately completionist and completionist (t) capes.