Vader and Sidious seem to be constantly dueling (plotting, counter plotting).

Palpatine with a lightsaber only works if he is hiding the fact he doesn't need one (he's like a shadow version of Yoda, who should hardly ever if ever use a light saber too).

Vader knows he doesn't need one otherwise he would have tried to have kill his master directly years before.

Besides shooting such a sequence would be very,very difficult to do what would it add?

Adding some spiritual advice from Yoda might be doable but once again, what would it add and wouldn't it just weaken Luke if he is getting constant Force tweets from his mentors?

There seems to be some sort of rules by which the Force ghosts can and can not offer assistance eg, "If you choose to face Vader, you will do it alone. I cannot interfere".

I imagine this extends to Yoda who didn't fare well when he last faced the Emperor.

Ghost said:

Should Coruscant or the temple be shown in the OT?

Once again, I think Ady may have some plans there so you never know...