I don't want to really suggest any ideas so much as give you things to think about...
1: I noticed you mentioned a tank stat, which confirms you plan on including tanking of some kind in this game. I personally find the concept of tanks in games to be really good. However there are levels of importance for tanks in games. Touhou labyrinth for example. places a medium amount of importance on the tank. For example, you can lose all your tanks and still have a fighting chance against most enemies (baal avatar? LOL NO U DED), but chances are much better for you if you can manage to keep one alive.
Then you have games where tanks are virtually title only. Pretty much any FF game for example, you can get characters who are tankier, and stick them at the top/front of the formation in hopes that they get smacked most often. But in reality, it really makes little difference where you put them, and you really can't count on them protecting anybody without a move that specifically makes them take a hit for someone.
Then you have games like your typical MMORPG, where tank survival is basically an absolute must, dead tank = dead party next turn pretty much. This probably is not very easy to accomplish well in a turn-based game without making the game piss-easy. But who knows. I personally would like to see someone manage to place more importance on tanks than in Touhou Labyrinth, without really making them all-important like an MMO. How this is done I can't really say. Touhou labyrinth is still good with what it does though. Seeing some experimentation within reason in this regard would definately be cool though.
2: You mentioned fixed MP values, I've never though about this, sounds interesting. I assume you intend to make characters able to regen mp like focusing in Touhou labyrinth? Or do you want a more item-based system with ethers and the like, or something else? I think giving the player the option to always regen mp at will at the cost of something (turns for touhou labyrinth) enables more strategy IMO, provided that you don't make spells requiring mana as powerful as it is in other rpgs. (Imagine if the Touhou labyrinth healers could heal as well as a final fantasy healer? Oh yeah cure 4, full hp for my entire party! woot, yawn). There are probably other interesting systems too though, where maybe you don't regen mp at all in combat, but always gets filled after every battle, or whatever.
3: Trash... This is important to an RPG, but it NEVER gets mentioned or praised in them.. IMO I think the trash in Touhou labyrinth went a long way to making the game great. It was challenging itself after awhile, and made exploring the maze fun. I know when I play most rpgs and I get attacked by monsters, I think "argh...sigh" most of the time. That's my personal opinion. However, every method of making trash memorable has its pros and cons.
Heavy-damage trash like F13 sword-fish are really frusterating sometimes. Ok so I start the fight against 2 of them, chen can't 1shot them yet because I'm not over-level, and oh look, they both slash-dived my tank before anybody else could move. Now I have to go back to town. argh.
I personally prefer trash that is tough and beefy. It can dish damage out, but it's not going to take a guy from full to dead without you being able to do something about it. It WILL however make you spend mp or else probably lose lives for it...I KNOW many people do not like this though because most people would rather move along and not bother so much time against trash. If your game is mostly story-driven, this is probably not the right road (as much as I prefer it >=( ).
4: Your idea of making each character use a different stat for their attack is good. I like it when casters use magic damage for their attack, and warriors use strength, etc. Touhou is a bit more difficult to do this with though since EVERY character is pretty much a character with magical powers. Maybe you should think about giving eery character the option of choosing which kind of attack to use (slap/stab/slash/whatever their melee is, magic blast, etc). This would also make characters with decent stats in multiple catagories more attractive.
5: customization. 6 party members allows decent customization alone via party selection. However how about character customization?
6: Character powers: Everyone knows that everyone in Touhou has a special power which they often don't use in the games (except Reimu's ability to fly, sakuya's ability to manipulate time, etc). Are you going to try and fit this into the game somehow? I think doing so could be fun, but very difficult to keep balanced. I think it would be kinda cool though if they added small effects. Like have Nazrin around, and get more gold per battle, or more rare drops, or something. Patchy could use any attack and not worry about elemental strengths or immunities by mixing elements in her attacks like she is perfectly capable of doing (probabily easier than give her 500 different spell element combinations >=P), Having remi in your party has a free "life 3" on one character once per battle, or something. Just ideas, none of those I'm sold on...Honestly though, with so many characters, the list would probably be huge, and having that many effects running around would probably bog you down alot. Maybe something to work on AFTER a release...
7: money/shops/items? I know when I played Xenosaga 2 and it had no shops I was upset, and I think it hurt the game... Then I played Touhou labyrinth and I decided it's not that bad. Personal request though, whatever you do PLEASE don't have "if you didn't do this at this point of the game YOU LOST THIS ITEM FOREVER!!!BWAHHAAHA". I hate those. People should be able to play a game WITHOUT a guide, and be able to get it all afterwards if they please WITHOUT having to start all over again, reading a guide before taking every damn step so they don't screw themselves over.
8: combat, typical console rpg combat? something more like Lunar? (
http://www.youtube.com/watch?v=SgfuiZly7FQ&feature=related), or something different still?
I'd post moar but I gotta hook something up for my bro, blah.