Lets make a check list of what we've gone through, and still need to work out:

:arrow: The theme is medievil, warriors, knights, wizards, etc?, with robots and guns and stuff?
:arrow: Total 3d, characters, maps, effects?
:arrow: Square Tile, Flat maps.
:arrow: Multiplayer, connect to a server to play games, and trade items?
:arrow: Open source
:arrow: Portable


and some other stuff to decide:
:arrow: Render engine? OpenGl or a custom engine?
:arrow: Single Player? MultiPlayer with computer controlled players aswell?
:arrow: Is game data saved on a server or users pc?

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Here's some of my random ideas :

Players start by creating a profile, and start off with a little bit of money.
They can go to a sort of 'Market place or town/city', and can buy/hire characters. These characters are slaves/loyal soldiers/mercenaries/minions and they work/serve for you. And they usually start off low level. You can buy items and equipment for them to use in battle.
When you've got a small party of characters ready, you can then go fight with them. Train outside in a 'wilderness', fight in an arena, or challenge another player. When you win battles your characters will gain EXP and increase levels. Giving the RPG part to the game.
As well as buy/sell/trade items you can also trade your characters. So you can train a warrior/slave upto a high level and sell him to another player, for money, or for another character.
In single player there might be some sort of story that you progress through by doing quests and stuff. And random NPC adventurers will ask to join your party through the game, which will help you in battles.

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Players could have 'Land', not physically but just a number. This 'Land' determains the max amount of characters you can have.
Say, you start off with 40 acres, giving you a max of 2 characters. You fight another player, who has 60 acres. You win the battle against him and you gain a portion of his land, say he loses 10 acres and you gain 5 acres (the other 5 acres got razed in the battle and became worthless).
You could also be able to sell off your land, to other players. Or even raze/dig up your own land to find items and stuff.

Well thats just an idea.