^Yes, I would have to say that, overall, the Gamera films have gotten better over the years, even though the last film, Gamera the Brave didn't do as well as the Heisei films did. I think Gamera had more to work with: less films and a mediocre Showa track record gave plenty of opportunity to experiment and break barriers. With Godzilla, there isn't that much luxury.
Godzilla vs. Gamera wouldn't hold for an entire movie. Like as not, the two may eventually clash, but I think that they would end up teaming together to combat a much bigger threat, most likely some Gyaos hybrid or King Ghidorah or even both if it came down to it. I can see Godzilla and Gamera clashing at first, but not for entire movie. That just wouldn't do well at all. King Kong going up against Godzilla was fine, and I think a rematch would be great, but you can't use that same kind of formula if you were to put Godzilla and Gamera in the same film. Gamera would inevitably end up being a heroic figure - the only times Gamera was seen as a threat was almost always due to a misunderstanding. Look at its track record:
Daikaiju Gamera/Gammera the Invincible/Gamera: The original film, and I've seen two different American versions, one being Sandy Frank's dubbing. Gamera was set up as a villain, although this is a bit debatable, considering it saved a child. Mostly Gamera was the villain only because of the damage it created, and it needed to be stopped.
Gamera vs. Barugon: Seen as a villain only somewhat, and being more of an anti-hero than anything else. Gamera is seen dishing out destruction early in the film, but when Barugon shows up, Gamera sort of becomes the hero, although not as much as later films would later depict it.
Gamera vs. Gaos through Gamera: Super Monster: Gamera is now full-fledged hero, and the only times you see Gamera treated as a threat is mostly from the antagonist's side. Only a few times has Gamera been seen as a potential threat: Gamera vs. Viras, when Viras has Gamera under a mind control and sends it to destroy things. Gamera is not a real threat, however, and perceived as such only because of Viras. In Gamera vs. Jiger, Gamera is only momentarily seen as a threat when attempting to stop the excavation of the Devil's Whistle on Wester Island, but, there again, it is a misunderstanding, and Gamera is only trying to prevent future events - and fails.
Gamera: Guardian of the Universe, Gamera 2: Advent of Legion, Gamera 3: Incomplete Struggle: In all films, Gamera is a bit of an anti-hero, but not the villain, although it is initially perceived as such in G:GotU. The only time Gamera seems like a villain is to Ayana in G3, and that is because of her perception of events surrounding the death of her parents and blaming Gamera as a result.
Gamera the Brave: Gamera returns to being a hero, and not perceived as a villain, although still a bit of a threat (self-destruction being the primary concern).
Truthfully, while Gamera offers a great deal in giving a person freedom to experiment, the character itself would be difficult to change. I cannot see Gamera successfully being pulled off as villain, to be honest.
Toho, however, would not want to have Gamera in the same film is Godzilla, unfortunately, as evidenced by Toho's continuous refusal to have the two together. I can see from a business perspective why they would do that, although, as a fan of both franchises, it is rather disappointing. If LP can pull off this upcoming Godzilla film (and I hope they do), then we may get a crack at Gamera possibly gaining entry through LP, although this is pure conjecture and unlikely to ever occur.
Now, as far as a 60th Anniversary film, that is likely to come into play, although with the LP project going on, I couldn't say if it would be done by Toho or if LP would take the helm at that point. Toho hasn't missed any anniversary films to date, so I would think that it would churn out a Godzilla film of its own just to celebrate that. The only question would be, after the LP film and the anticipated 60th Anniversary film, where would Godzilla go from there? Dependant upon how either film would do, Godzilla may make more comebacks, or be reduced to anniversary films. Either way, I do believe that we'll get a 60th Anniversary film, and hopefully it won't end up like GFW.
There are as many people as there are stars...
There are as many encounters as there are stars...
-Ruri Hoshino, Nadesico: the Prince of Darkness