Mace will never be a force spirit; he did not learn the secret of immortality taught to Obi-Wan and Yoda by Qui-Gon. According to Lucas, they "caught" Anakin on the other side, although I'd much prefer it if Anakin just dissapeared after his death and it was assumed since he was the Chosen One it was natural for him to become one with the force. What I would like to see (I've posted this before), is this:

-Luke looks off to the distance.
-Old Obi-Wan and Yoda are there, and old Anakin appears.
-Luke smiles at them, and Leia comes over to him ].
-Old spirits transform into their prequel trilogy selves, dressed appropriately (no cut-and-paste jobs).
-Luke and Leia join the rest of the group.
-Qui-Gon appears beside them and the group acknowledges each other.
-Final shot of the OT characters.
-NEW final shot of SAGA characters, the spirits.

Since the saga is really about Anakin, the final shot should feature him and the other full-saga characters. That, and I never liked that last shot of laughing Leia, clapping Lando, and dancing Ewoks. A new, triumphant musical cue should also be added to end the saga with an emotional bang. It would be nice to see Padme there with the rest of the spirits, to have Anakin reunited with his love and Luke and Leia see their mother, but the logistics of explaining how that would be possible would outrage many fans I'm sure. In my opinion, NOT seeing Padme really weakens the saga; what kind of happy ending is it when we're left to assume the hero's love is lost to oblivion? All that aside, seeing Qui-Gon makes perfect sense as he is the reason the rest of them are there in the first place.