So lets talk a moment about the differences between ORO and ER-ORO
1. Progression
Oro would work in a sort of "Branching pathways" design. Ie one entry point and it expands from there. Tree like. ERORO, where the city is now a malfunctioning auto colony, would have a lot more room for different paths in. Because the game wouldn't have a central goal of "killing" the colony like in ORO, I wouldn't have to worry about obfuscating your path or building layers of difficulty. Rather certain areas would be difficult and you can choose to go there or not.
2. Malevolent landscape vs apathetic one
One of the major differences here would be that ORO's world is literally out to get you. The city is alive and it hates you. Being in the City in ORO is like being a microbe in a living body, you are a target of everything that body can throw at you. ERORO's city isn't alive in the same way; it's alive due to the auto nature of it, but it doesn't hate you. There can be civilizations or at least groups of beings or things living in there that are neutral, friendly or hostile to you. The best way I can think of to describe it is that it would be something akin to an alien ecosystem, rather than a death trap.
3. Goals vs Desire
In ORO you have a definite goal, Kill the City. In ERORO it would be much more about personal goals. Missions would come in the form of Requests that are made to the Merc population of the bar. Bounties, hired guards, exploration requests, all those kinds of things could be asked for. And whats more, players would be able to create their own requests if they wanted, hiring other players to work for them. If they felt like it.
4.Melee vs ranged
Simply put, ORO would involve my attempts at a more in depth melee system with weapon range, damage type and speed, while ERORO would be based on a more universal system since it would be a mix of lots of different kinds of weapons, many of them ranged.
5. Minor missions
One way I've though of to handle massive amounts of people is to create what I call "minor missions". These would essentially be CYOA style missions, where you would get choices and rolls based on those choices. Ie you would be given a situation and 3-4 options, each of which represents a roll. You could choose the roll you and your teammates want and then do it. Depending on the roll (probably more pass fail than true RTD) you would continue the story until another roll. This would go on for roughly 15-20 turns and then would end, with the number of successful rolls and choices determining your outcome. They would of course be more simple than major missions, with less freedom, but they would give you something to do and a way to make some money or get some items while not on a major mission. This could be done for both games.