Question: Does anyone know why Gambit was featured less and less in episodes as the show went on? He was one of the most popular characters in not just X-Men, but all of Marvel Comics around the time this series first aired, so unless it had something to do with his voice actor's contract, it seems odd they wouldn't have utilized him more.
Answer: Chris Potter, the original Gambit voice actor, did indeed quit the role in the 4th season. His last episode was The Phalanx Covenant Part 1 so it seems reasonable that Fox would limit the use of the character even though it was recast. It doesn't appear that Potter left the role due to animosity, he stated in an interview that he wished to play Gambit in the first live action X-Men film.
Question: In all the Highlander movies and in this show, was there any reason given why immortals can't fight each other on holy ground?
Answer: In the episode "Little Tin God," Joe Dawson mentions a duel between immortals on a temple in Pompeii—just before Vesuvius erupted. Whether the two are supposed To Be connected is up for debate, but still.
Answer: Not directly. But there is a penalty involved. In Highlander 3 the fighting on holy ground resulted in the destruction of one of the weapons. In End Game several immortals are murdered whilst on holy ground but no repercussions were ever revealed, so it's possible it is specifically about the fighting. Nobody knows what happens when an immortal is killed in a duel on holy ground but it's safe to assume the penalty will be severe because all immortals respect this rule, even the most evil ones (except that one moment in Highlander 3).
Answer: Because he stole a chemical from his lab.
But Batman knew that with it, Clayface could permanently retain his shape and Clayface desperately wanted to be Matt Hagen again. In a sense, Batman stopped Clayface from becoming human.