There have been debates going on since Season 5 of My Little Pony Friendship Is Magic. In "Crusaders of the Lost Mark", Diamond Tiara reforms after the Cutie Mark Crusaders help her stand up to her snobby mother Spoiled Rich. In the second part of "The Cutie Re-Mark", Starlight Glimmer reforms after realizing the mistakes she made with alternating the timeline when Twilight and her friends got their cutie marks. After that, Starlight became Twilight's students and has been as a recurring character since. But Diamond Tiara hasn't had a single major role in the MLP: FiM and was reduced to background character with no speaking roles.

The chances of the idea are slim in Season 8 (since most things have been leaked), but there's a possibility for Season 9 to happen. The idea is if Diamond Tiara meet Starlight Glimmer she would want Starlight to take her under her wing and act like a big sister. This thing occurred with Scootaloo and Rainbow Dash since the Season 3 episode "Sleepless in Ponyville". If this would work out, then why not have Starlight Glimmer and Diamond Tiara act like sisters.

Let's get this to happen before Generation 5 starts.

By the way the picture used in the petition belongs to deviantart member BrokenHells : https://brokenhells.deviantart.com/art/Starlight-Glimmer-and-Diamond-Tiara-Request-585006240  (I just added that as an update to let people know)