There's various definitions of RPG out there now that most games at least have 'RPG elements', and many styles that can't really be mashed together into one game. If I would go down in history for one achievement in gaming though, I'd like it to be creating a strategic battle system with a diverse move set* that can clearly show players everything that is happening at once in a battle showing all the effects and what they mean, without being turn-based or overly-automated like Final Fantasy XII/XIII.
Once that's done, tack it onto an open-world Feudal-style game with Onimusha-type artwork, though lacking in oriental stereotypes. The thing to truly set it apart is that its various warlords will battle and conquer/destroy each other's cities and castles regardless of your actions, though your taking a side can tip the balance either way. After a predetermined period (say about two years of game time), a final battle and ending will be selected depending on which faction you've sided with and how successful you were. Then, after you win, it resets the story back to the start and lets you keep your stats and equipment, allowing you to make a bigger change each time in search of the best ending, as well as possessing the knowledge of what happened last time.
So you can stop warlord A from poisoning a river to decimate the troops of warlord B along with all the villagers who depended on that river and died from it in your previous playthrough, burn down a city that survived a siege previously, kill that annoying merchant in village C who ripped you off, etc. Sort of Majora's Mask on a larger scale, each time you are more powerful and can effect the outcome of more battles/events. After several playthroughs, you will be impressively powerful enough to become a warlord in your own right if you wish to. In addition to all this, there is a race of demons/genma acting more like traditional CRPG enemies causing trouble in villages or trying to make the war lead to even more deaths to feed them, but they come from a hell-analogue that is the bonus dungeon. Perhaps have the excuse that at a certain time, the demon king climbs from the pit and resets time unless they are killed by you in the best ending. Which requires a powerful army willing to follow you down into the depths to begin with, or a party of powerful players.
Finally, add Dark Souls-style player interaction that lets other players hop into your current realm and either help or hinder you as they wish (can be turned off completely if you want a fully single-player experience), and you can do the same.
*- As in every attack/ability common to CRPGs and many more. Since this is already an anachronism stew, we allow primitive guns to exist alongside katanas, spears, bows, axes, sais, flails, and magic.
