I'd say the three most likely game modes to return would be Tower Defense, Wing Rush, or Bingo.
Tower Defense had an active and dedicated community up until it's removal.
Bingo is commonly requested to return, especially by OG players. However the vast majority of Cubecraft players never got to play it (including me).
Wing Rush is a fun mode, but I think it has an ace up it's sleeve. Wing Rush is based on Glide, which was the most popular minigame on legacy console. But no large server has Glide anymore, it's an untapped market. Unless you count Terra Swoop Force, which is free on the marketplace.