I wouldn't be against Scar appearing as a ghost at all. I can well imagine a scene with Kion meeting Scar's ghost being very epic, if done right. It would definitely need to be something else than his spirit returning to cause random trouble for one episode, though. It would need to be part of a bigger, more emotional story that goes on throughout the series. Such as Kion struggling with the Roar and the ghost of Scar eventually appearing in some episode and trying to persuade him into using it for evil. He should be an ongoing presence in the background, just like Mufasa... but the opposite, heh. Scar is one of my favourite villains ever, so seeing him again in any form (whether as a ghost or in a flashback) would be absolutely amazing. I must admit I do have some hopes for that happening. The Lion Guard might be the last chance we ever get to see Scar again, so I'd definitely take it.
I'm not too terrified they'd butcher the character. Even if they did, it wouldn't change anything within the original movie. And I wouldn't be too worried about a different voice actor. They did a great enough job with Zira's new VA that I have faith they'd do it right. Besides, while it's true they'd NEVER get Jeremy Irons back in the role, they could probably rather easily get Jim Cummings. And I'd have NO issue with that. The fact that people still don't realise he entirely sang "Be Prepared" shows how close he can get to Irons' voice, and that he could easily take over the role without a complaint from anyone. Not to mention that if it was Scar's ghost, there'd probably be a filter and effects on his voice anyway.
I actually wouldn't mind if his voice wasn't exactly like the original, anyway. To be completely honest, I was never the biggest fan of Irons as Scar. I frankly think Scar has a better voice in some other languages. The Finnish dub of the Lion Guard actually has pretty much all the same VAs as the original movie had, so if Scar were to ever appear in the show even for one scene, and our dub could get his original Finnish VA who I grew up with, I'd probably cry lol.
That voice is such a big part of why I love Scar's character and thus a huge reason why I love the original movie so much.