Is a scheme in which the characters have power and then lose it feasible? The idea I had initially was that most of their powers were consumed in a temporal irregularity. In order to have a game featuring the same people without powerup-whoring everyone to unbelievable levels, we have to obey the Yuna Rule somehow.
I would like other people's opinion's about this idea. It occoured to me too, but at the same time, I dismissed it as entirely too cheesey, as it seems too convenient, if you think about it. But that's just me, but it caught my eye, because I thought the same thing (though, like I said, I hated it as soon as I thought it). I like the idea of learning everything over again (like at the 3 techs each stage).
If you're intent on doing it, there is a precedent: Magus lost most of his power at some point between his defeat at his lair and his decision to join your party late in the game.
But I agree; too cheesy. Instead, perhaps take a little artistic license. I know something about trained skill versus innate talent: You have to work hard to maintain the former, and the sharper you want to keep your edge, the harder you have to train just to hold the line. In Chrono Trigger, Crono & Co. were fighting very hard for months, if not years. Moving on with their lives in peacetime, they would have had other priorities and it would have become impossible for them to keep that sharp a combat edge. So, to me, that would suggest that we start Crono, Lucca, and Marle out at a lower level, and with fewer spells, than they enjoyed at the end of CT.
But at the same time, let's remember that these are not innocent people anymore. Crono can only have that boy-becomes-a-hero adventure once. In Crimson Echoes, he is already a hero. He's got blood on his hands and experience under his belt. Surely he and his friends have not lost
everything they learned. It would be much more realistic to start them out at a moderate level, rather than Lv. 1, and to allow them to retain an assortment of their lower magic spells and tech skills for immediate use at the start of the game. Super-proficient skills like Luminaire and Flare can be lost, just like Magus lost Dark Matter, but everything up through Ice/Fire Sword seems justifiable given the amount of training that we can assume Crono and his friends
did maintain.
This points to starting out the main trio somewhere in the Lv. 20 range, with selected choices from the lower half of their magic talents still available. This can be balanced out by harder enemies from the onset, and the creation of a few new spells to flesh out the new growth curve.
As for characters who join later in the game, their skills would have to be sufficient so as not to imbalance the combat. That's just how it has to be, whatever rationalization it takes.