They already did. It's called Chrono Cross
Here are the problems with Chrono Cross:
-It's too big for its own good. 50+ characters? Do we need 50+ characters? Every character is practically the same. One of the things that was so amazingly great about Chrono Trigger was you had this core cast of characters, each one was extremely unique and had his or her own unique abilities. So it felt like you could really design some great teams by just changing up the characters. Each character also had an insane amount of story development which made you care for each one.
-No Techniques - well, I take that back. I think there was *ONE*. If you got Glenn (which forced you to not get another character I think!) he could do a technique. But I never bothered. That is so lame. One of the most awesome aspects of Chrono Trigger was the ability to combine spells with other teammates for double and triple techniques.
-No real time travel.. I think there was some here and there, but it was generally not in the game. Time Travel was one of the things that made CT so great, because it was awesome just to see how much the world changed, or how your actions in the past would change the future. That was one of the cool things about that game.
-Bastardization of CT story - once you even *get* to some connections with the CT story in this mess of a unneedlessly complex plot, every instance is like a smack in the face to CT. It's like "Well Robo died." "Well Chrono, Lucca and Marle are dead". "The kingdom of Garnia (sp?) was overthrown". etc. And then they have this huge big reveal that Lavos (sp again? I havn't played CT in awhile ) was actually the cause of Humanity's birth. It's like, gee, thanks. The thing that was the worst bad guy in the first game is actually the reason why I'm here at all. That's nice to know. I'll also add to this the utter lack of locations from the first game, to the point where I couldn't even tell where the heck I was in the game world. It was extremely confusing, and I almost think the game was indicating that the CT world was like an "experiment world" or something and the CC world was "the real world". It was so stupid.
-No Dragonball Z artist - say what you want, but that art style made the whole game.
And that's only what I remember, keeping in mind I last played this game like 6 years ago.
Edit: BTW, CT was the only game I can think of (at least at that time) where the main character actually *dies* halfway through the story. That was huge.