Reading some threads about LIGHTSUM made me think that Cube is secretly reworking their concept. People may have forgotten but Chowon (a member in LIGHTSUM) was part of the most viewed, and some would say, the best performance in a Produce or Produce inspired survival show.
Here's the facts. Cube got rid of all the underage members. LIGHTSUM is apparently still active on SNS but they haven't done a comeback in almost a year. AND according to some screenshots I saw someone posted, Soyeon says there's Cube trainees (not sure if male or female) are preparing to debut. This could be one of few possibilities. Since LIGHTSUM is doing cute concepts, they would obviously have to mature their concept now that they no longer have underage members and a younger sister group will debut, so they give them the cute mantle.
If LIGHTSUM comes back with a Rumor like concept it will be something a lot of people would be looking forward to. I would love to see Juhyeon do a concept like that.