Mauron, TheMage, and I had an idea for a hack a while back that followed a group of misfit time-travelers from the OVA (a Nu, a Kilwala, and Johnny, amongst others) in a sort of Rosencrantz & Guildenstern Are Dead behind-the-scenes look at the events of Chrono Trigger.
Essentially, these heroes are either in a time era mucking things up right before Crono and friends arrive, or right after -- regularly crossing paths. The only caveat is that we hoped to have a fully explorable 1999AD and the party would be present for the Day of Lavos.
Overall it was meant to be a comedy/drama with a bit more comedy than Chrono Trigger, while also offering a bit more darkness (for example -- the lead character is the daughter of the Director from Trigger's Bad Ending and is a literal mute who, originally from 1999AD, witnessed her mother die during the Day of Lavos and tragically blocked the memory due to the trauma).
The cool thing about it was that we literally could use the exact same tilesets, enemy placements, etc., and had to build very little in the way of new assets. Even many of the coded events could be the same, just edited a bit to match the new characters.
We did toy with doing away with both Ice Age era's, replacing them with a not-quite-as-bad-as-2300AD version of 2100AD, and a fully explorable 1999AD. Looking back, however, I think losing the Antiquity era's would be a huge mistake.