Given that TLJ builds on the prequel trilogy, in details as well as themes, I think it’s reasonable for Ep9 to appropriately tie into the PT and OT to attempt to conclude the entire story. I don’t see a problem with Anakin appearing.
Even if you want to ignore the PT and SE, it’s not like Sebastian Shaw is available. Perhaps they can depict Anakin as being a youthful middle-aged, about how old Hayden is now. In my book, I’d suggest making him appear vaguely elderly, but not necessarily a Shaw lookalike. To look timeless, in keeping with his ghost in ROTJ somewhat, just like how Ben and Yoda didn’t appear youthful.
I still think it’s profoundly dumb to insert him into a 1983 film the way they did, and would always prefer the original regardless; a new movie with Hayden reprising his role wouldn’t change that. What if McGregor appeared as a force ghost? Would you resent that, too?
Anakin could also appear via a hologram. If Artoo still has Leia’s Help me Obi-Wan message in his memory, there’s probably lots of other things in there too.