In order to sell, you need something to set your game apart from other turn based strategy games that are out there. Will it be like early Final Fantasy games? I've played fallout 2 and it does set itself apart from others with it's gameplay.
There actually aren't many of this type. This RPG is controlled like a turn-based strategy, similar to Fallout and Temple of Elemental Evil, NOT Final Fantasy or Pokemon. You can (and must) move your characters in combat, and the environment does matter. (For instance, the game takes cover into account, standing around naked in inclement weather will inflict cold damage, and you have to make swim when in water.)
With whatever you're doing, you need to add an aspect of it which sets itself apart from other "cookie cutter" turn based RPGs. Maybe something like customization of armor and weapons like Diablo where you can add skills or gems or however that works.
To some extent, I already have this through the modification/enhancement/enchantment system. Crafting is also a major part of the system, and there's an entire class devoted to it. (Artisan.) The game includes a lot of features that look exploitable, allowing players in any class to create apparently overpowered builds. (Like combination that allows Savants to gain something like 600% experience, or the one that can let a warrior hit enemies over a kilometre away, or the one that can allow casters to cast all day without emptying their spell slots.) And, of course, there are two other important features. There aren't many cooperative games in the genre, (up to 8 players here) and there aren't any that I am aware of that allow you to have quite this many characters. (Total 16 player characters, and 32 total characters in the party. Even in single-player.) Customization is also important, as the system has a large number of feats, traits and templates the player can use, in addition to having twenty classes, twenty species, two sexes (no shit) and five ages for a total of 4,000 combinations in basic options alone. Adding on the ability to make sixteen player characters right off the bat, and create a custom home village gives this a great deal of weight. You can even set up party relationships in advance, and set up relationships to your NPCs in the custom village. You can not only decide that, for instance, your warrior is married to your shaman and your ranger is their daughter, you can make it official when making your party and it'll impact in-game dialogue.
The last thing is the ruleset itself, which is farther on the "simulation" end of the spectrum than any game I've ever seen and contains a number of features not common in this or any genre. A character's wounds impact their body, not just their health, and limb damage causes impairment. Low health also causes penalties, and when wounded a character bleeds. (Which does, yes, equate to more health damage.) Wounds that are not healed can become infected, and your needs must be maintained. Death in this system can be an extended, bloody affair and if you can't do damage control you're pretty thoroughly screwed. (Translation: Get a healer or bleed to death.)
Do you have a story written out?
Yes, actually, I do. But it's a bit twisty and I don't want to give out any spoilers. The working title is "Nonnullus delusion." (Might just shorten that to "Delusion.") It's part of the motto of a faction within the game, which crops up a lot throughout gameplay. "Populus nunquam redono suum licentia tamen sub nonnullus delusion." It's there for a reason.