She's very popular. So I could see them wanting to put her into pony in G5 at some point, and her fans would like to see her in G4, which is always money in the bank (Hence why she's going to return to equestria in the upcoming EQG plotline and talk to celestia before returning to the human world to fix the problem).
And she didn't exist to take twilight's place, they didn't just excise twilight from the series; She TOOK twilight's place as central leader, but that's more because they were able to Manipulate human twilight's role to be different and sunset already had some momentum.
Honestly, I DON'T want her in G4 pony simply because she can't really work well without being redundant. They were able to alter Human twilight and her focus to allow Sunset to work well, but that's not really an option in the pony world, especially with only S8 and S9 left.
I fully 100% expect her to show up and join the group in G5 as a mane seven, after serving as the dragon to one of the villains (Like Nightmare moon) for a season.