So, Return to Oz is pretty popular for being a faithful representation of the Oz books. This may be a matter of debate, but it's generally considered a well-done film. However, there's some characters who didn't make the final cut...
"Wait!" I hear you say, "The Tin Woodman and Cowardly Lion were in the movie!" And yes, they were. But an earlier draft of the story had them having a much larger role. Either they hadn't been turned to stone and escaped with Dorothy, or were at the Nome King's Mountain with the Scarecrow and not transformed with him, and later got to try their hand in the ornament rooms.
Personally, I think it was a better decision for the movie to cut these characters. As it was, the production had gone overbudget, and having these characters at the Nome King's mountain would have made for a much longer sequence and a more frantic restoration.
Those who have read Ozma of Oz will remember that Dorothy and Billina and even the Nome King are unable to locate the transformed Tin Woodman, which is referenced by Tik-Tok not being restored before they leave the Nome King's Mountain in the final cut. This let them pay tribute to the original text and also let them not have to handle the Tik-Tok puppet during the chase scene. Now imagine if two more complicated characters were added to the mix. (They already had Jack and the Gump.) If they'd gone the "We didn't find them, oh look, here they are!" route with them, doing it more than once would strain credibility.
As for the army, more characters would have crowded the chase scene, so it seems that this is a "less is more" situation. Less characters was more tension and suspense as well as focus.
Return to Oz drew most of its plot from Ozma of Oz with The Marvelous Land of Oz being close behind it. Those books had a few more featured characters who don't seem to appear in the film. Well, the Sawhorse does appear. As an illustration in a book a woman in the Emerald City was reading. It might have been nice if he'd been included in the return to the Emerald City sequence in the finale, but that would have required them to build another creature that would barely have any screentime.
In The Marvelous Land of Oz, Ozma had been raised by Mombi (who was an old sorceress in Baum's world; Return to Oz mixed her with Princess Langwidere, creating a character that was a little of both but ultimately neither), but she was transformed into a boy named Tip. Return to Oz had Emma Ridley (who played Ozma) appear in Kansas as a mysterious, anonymous girl who rescues Dorothy from the clinic. It's suggested by dialogue at the end ("I was afraid you'd drowned," Dorothy says, to which Ozma nods knowingly) that she is Ozma.
I have a theory about this: a dancing girl says that Ozma was enchanted into a mirror. As such, until Dorothy frees her at the end, Ozma only appears as a shadowy form in mirrors. If Oz is a mirror version of Dorothy's world, then wouldn't it make sense that Ozma is suddenly able to cross over to Kansas while she's under this enchantment? Or, considering Ozma appears in Dorothy's mirror at the end, maybe this is just something she can do and Mombi's enchantment let her project her entire body into Kansas.
So, if they'd used Tip, would a boy have rescued Dorothy and adventured with her to Oz? Perhaps. But unless they'd come up with a good dynamic for him and Dorothy, then we'd run into the "too many characters" problem again. And how could they pull off the restoration?
Okay, seriously, though, where was Glinda during the events of Return to Oz? She doesn't even get a name drop. Now, possibly she was the one who helped send the key to Oz, but you'd think that even with her usual hands-off rule, she might make a concession when the fate of the entire Land of Oz is at stake.
In Sam's Return to Oz comic adaptation, Glinda makes an unannounced appearance and declares who Ozma is. My only theory is that she was attempting to get Dorothy to Oz and help free Ozma just as everyone was turned to stone, deciding that if she was successful, Dorothy and Ozma would restore her along with everyone else.
So, who knows how Return to Oz might have been if they'd added more characters to the plot? But I think it works well as it is in its final cut.