Aha, too easy! Since I've had to go down this road for...something else...
Pre-65000000 B.C. -- Something fantastic, like the Cambrian explosion, that long predated sentience. We'd see the makings of the early humans and Reptites, and perhaps even find a Nu or two. And to mess something up here would ruin the entire future, so that drama would be immense.
13000 B.C. -- I'd like to see Zeal on the ground. It must have been an interesting place; they might have devised a way to magically keep the storm at bay while constructing elaborate cities. By 12000 B.C., they simply decided to take to the skies to avoid the storm.
3000 B.C. -- What wars, tribes, and heroes existed before the Kingdom of Guardia? Surely there is an entire unwritten record out there of Gilgameshes and Great Zimbabwes.
1 A.D. -- The founding of Guardia. But Crimson Echoes will explore this era to the hilt, so you will get to see this one.
1400 A.D. -- After the industrial revolution, but before the domed cities, there was a middle period. How did the Porre / Guardia battle play out in the end? What's the Guardia lineage doing at this point, and how do the citizens feel about prior history?
1999 A.D. -- So, let's see Robo and Johnny in their natural environment soon after their creation! Let's race Jet Bikes and explore domed cities.
But to break the chronology...
1023 A.D. -- Chrono Cross is history, but the global battle for power has just begun. The power of Porre is limited -- there are other strong countries, and even Guardia exists in some form. Meanwhile, the Acacia Dragoons of El Nido may have secured their island fortress once more, and are poised to strike with the added strength afforded by their noble ways and values. Furthermore, the heroes may still be alive. Cross could lead into some very exciting global issues; you can only wonder what Kato had in mind for Chrono Break.
~
But obviously, I'd want to return to Zeal again.