According to some sources, Zeltrax's real identity was supposed to be Billy Cranston. Yes, THAT Billy. But they couldn't get David Yost to return, so they made up a new character and played Remember the New Guy with him.
Other sources take the above a step further by saying that Billy wasn't even the first choice the creators wanted to be Zeltrax's real identity. Wanna know who their first choice was? Jason. Arguably, this choice would have been the one with the most impact. Considering how close Tommy and Jason were in previousseasons. However, none of these have been confirmed as possibilities by the actual writers.
Word of God: The ZeoChristmas Episode showed Tommy and Katherine would be married in the future, but her not appearing in this season prompted fans to question that outcome (not that there hadn't been questioning for years prior to that, of course). When asked about this, though, Doug Sloan stated that Tommy and Kat getting married was still in-continuity.