I never noticed the training option. Wierd. I think sanbondo could start a new series with TD as its start, use existing character who kinda rally up to solve TD, use chars of all factions for 13.5, with a chose your own faction thing. 14 could be a similar idea, with charging the needle castle with existing units and TD units as allys. The same for 15 and 14.5. The only thing is having every unit being playable would be dumb, so the returning units would be the units who are playable, mabye the 2.0/2.5 like Cirno, Rumia and co., the 3 fairys, Prismrivers, Keine, Shou and the other Buddhists could work as well. Later down the line, the other 3.5s and 2.5s that don't have a reason to join, Remi, Flan, Yuyuko and Yukari (though those two could quite easily be playable from the start of TD) could join in and eventually assemble a huge cast again. End this new series is with mabye another OC with Junko and Hecatia or mabye the casts from 16 or 15.5 joining in for the last game.