A battle system where you have to manually find out the range of damage you can do to an enemy before you can attack it. You can find stuff out about said enemies through questing or talking to npcs, or just general information swapping with other players. Involves basic math.
For pvp, when you click on the player you want to kill, you both get a short screen, showing the other player's equipment and their stat build (Like RO). Catch is something like you have to recognize what items they're wearing, and have to remember the hard defence bonuses said equipment totals up to.
Ignoring finding out the stuff, gameplay would look like... I dunno. Flyff? Or hell, RO.
For random numbers off of my head, basic physical damage calculation would be ((STR + DEX/4 (round down if decimal) + Weapon Attack Bonus) - (Enemy AGI/2 + Enemy Defence)) minus/plus x. Enemy defence is simply VIT + Armor Bonuses. X is a random number from 1-25.
So assuming you're wearing a Claymore which gives +180 WAB. Your STR stat is 70, and DEX is 60. Enemy AGI is 80. VIT is is 40. THey're wearing a Full Plate which gives 10 ADB.
So it'd be (70 + 60/4 + 180) - (80/2 + 50).
265 - 90
Damage would be 175. Then you add or subtract X, which is randomly chosen by the game engine, or whatever. But you only need the number 175 in order to attack. Skills are pretty much the same, but with damage multipliers and whatnot.
Idunno, I thought this would be fun...