To anyone aiming to make a JRPG, I would start off by recommending (possibly demanding) that they play a few specific games and really try to analyze what makes them work so well.
Pokemon, preferably one of the newest ones, the series has what is undoubtedly the single best balanced battle system of all time, play the game and go to any pokemon wiki (I prefer Bulbapedia) and just read about the types, moves, different pokemon, abilities, etc…
Golden Sun, the battle system is very solid, and is only improved by the quality animation and speed (both help keep it from being monotonous, I’m looking at you PSX Final Fantasy games). But most important, what would be a solid but otherwise mostly unremarkable battle system is made one of the most brilliant in the genre by the magic splash damage system.
Breath of Fire, while not specifically for the battle system, BoF deserves to be on this list simply for the menu system (in-battle, overworld, and especially in shops), far too many RPGs have everything overly complicated, but give no information to the player, BoF helps the player make the most informed decisions out of any RPG I’ve ever played, not once did I feel like I was ripped-off in a shop because I didn’t know what something really was, or was a perfectly good item because I had no idea how it was supposed to be used.
Now for my more specific advice:
Make sure that the turn-based battle system is good AS a turn-based battle system, not just an action battle system that has been cut up into a turn-based one. Real-time action battles can be enjoyable even when the player is essentially doing nothing more than the same basic attacks over and over, turn-based systems very rarely can.
One way to do this is to avoid lots of useless techniques, many RPGs give each character lots and lots of special combat options, but most of the time all but one or two are essentially useless. The pokemon games do this very well, with the limited amount of moves a pokemon can have and the extent to which the effectiveness of any move changes in different contexts, a certain level of conscious strategy is always necessary when battling enemies anywhere near your own level.
Similarly, outside-of-battle strategy can be deepened by making equipment much less linear. In most games there is a single progression of weak equipment to strong equipment, no thought is involved in choosing what to upgrade or buy outside of money management, any strategic options are usually limited to special accessories and kept to a minimum. Again, this is similar to pokemon moves, since any pokemon is limited to four moves and each pokemon has so many options, building a good moveset involves a lot of careful planning, choosing moves that work together well (with each other and with your other pokemon), that cover as many type weaknesses as possible, that maximize effectiveness against specific types, that are built for specific strategies but allow for improvisation… If something similar could be achieved with equipment it would add new levels of depth in ways few other RPGs have.
Make sure you play to the unique strengths of turn-based battle systems, things that cannot be done in real-time, such as allowing the player to take their time for strategizing (this can be helped along by making strategies involving thinking ahead and making abilities that work well to complement each other), and the ability to use multiple party members (again, like pokemon where many different pokemon can be used in conjunction to increase the effectiveness of each).
One more thing I feel I should mention is grinding. Grinding gets a bad reputation that is largely undeserved (MMOs aside), I’ve seen many articles and blogs about how grinding is all a big ploy to artificially extend the gameplay time of a game (and in most MMOs, it is) and call for an end to it, but they seem to be missing the true point of grinding. As a manga/anime fan and martial artist, I find grinding to be not a necessary evil like many, but an enjoyable part of the game that is simply poorly done most of the time (and I also prefer the term “training” because that’s what it really is). To keep it from falling into the traps that most RPGs succumb to, it must not be treated as a mindless “press ‘A’ repeatedly while walking back and forth” thing, but as a learning experience to train and challenge the player.
Training should allow a player to feel a growth of power (at a reasonable rate), but for the fullest effect, should not allow the player to really realize just how strong they’ve become until they’re back into the “main game” (no taking months to gain a single level coughmaplestorycough, and have you ever noticed that in manga/anime the characters actually get stronger faster the higher “level” they are? Why is that never true in RPGs?). Training should be dangerous (don’t let the player grind on enemies 50 levels weaker than them) and challenging, make the most grindable enemies tricky ones that require constant effort to defeat, provide handicaps that increase the exp gain. Training should be a learning experience, force the player to use different strategies than they normally would, and it should be where players gain most of their new abilities and learn how to use them.
