If not then what should have happened to him? Discuss.

Though I do support rehab. It would be better not to outright redeem him without any context, backstory and development. Though for that to work we would probably need a bigger threat or a Philosophical powerhouse of a person to sway him.

7529054
Yeah, he did not get any proper development in the season so reforming him would be too sudden. But that was because of the twist, if they decided not to do it then they could have develop him further and give him more depth.
I even think reforming the diamonds from Steven Universe could have worked if they spent more time developing them instead of pointless filler episodes.

7529070
I agree. It's really about time and pacing.

Grogar himself would probably carry a different backstory then his original incarnation, but going off it.

No, the original Grogar is basically a King of a dead empire-literally, the entire population is trapped in a pseudo purgatory with him and the city itself-and is a necromantic force.

He's driven by spite and vengeance, wanting to see his empire rise again, and those who destroyed it left in it's place.

He may accept a truce and treaty given certain requirements are met but he certainly wouldn't back down with remorse given the origin of his driving purpose.

Notably, like one would expect, we know nothing of Tambelon or Grogar prior to their actions against Ponies G1 or otherwise, so yeah, could be something there to change his mind but certainly not "redeem" him.

7529049
Only if it wouldn't come at the cost of a trio redemption. It would have been interesting to have him take more of a neutral route, maybe failing to take over Equestria, but perhaps being allowed to bring back Tambelon as its own kingdom in exchange for peaceful relations. Maybe have him be a complete villain to everyone, but still someone who loves his city and would take the humiliating deal to save it. Ironically, that could lead to him being a good example for Chrysalis, who could also reform because of that.