Your game in itself has a very addictive base play. And I bow to your success. However, I can see people getting tired of the same old structure too soon. So what I would suggest is a multiplayer with following features:
World Domination
> Players can create a faction (country/clan/group/order,etc.) with their own customizable flag which when built creates an area of diplomacy. And as discussed in the forums earlier, have a diplomacy window where you can set players on the Friendly/Neutral/Enemy list.
> Make the world almost limitless, meaning it would take you several hours in a vehicle to make a complete loop through the entire map.
*Flags - Can be built in different sizes, from very small to very large, with the smallest requiring the least amount of resources and the biggest of course the most resources.
- Smaller flags capture small areas, while larger flags capture larger areas.
- You have to have a flag area to be able to build any kind of military structure. Everything else can be built in any area.
- Anything that is built outside of the flag area can be modified by any other player. Build within your own flag area to lock it from enemies or neutral.
*Friendly - players on this list can help build/repair/harvest and function anything that is within the area which the friendly faction possesses with its flags.
*Neutral - Players can walk through and view and interact with other players within that flag area without being able to be shot and killed by other players and/or turrets/robots/etc.
*Enemy - Players with enemy status will be shot at by defense systems of that faction and players in opposing factions can also shoot at each other.
*Diplomacy system where players can request to join a faction or declare war. Or just remain neutral/freelancer.
*Death - When a player dies, he can respawn at his own flag area/spawn beacon/etc., however, he cannot engage in combat or build defense structures for a set time (5 mins for example). He also remains invincible for that time.
This allows for a whole new open world of conquering land and defending against invasions. Players can join factions or create their own. Neighboring factions can build armies of people and defense systems, or a massive invasion army, robotic soldiers, mechs, conquer neighbors and replace their enemy's flag with their own and take over all the area of that flag including all of its machinery and resources. There are many other details you could build upon this system, like an economy of buying and selling resources, paying wages to players so they can buy upgrades for their character to become the ultimate fighting machine and earn even bigger wages, and so on. But for now the above would be a great ground structure for a MMO game that could bring in masses of players, which would also increase your pay
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