I have been playing with this little project on the side for the past few days now. It is called Monster Rancher. I'd like to say right here it isn't supposed to be a brother to the real Monster Rancher game. Players will be able to find monster eggs, which they will have to carry around to have them hatch. Once they hatch the monster is automatically bounded to them. The monster will only do what that trainer asks it to do and no one else.

With the basic rpg like aspects of the game there are also other ones. You will need to keep your monster well feed and make sure it doesn't get sick. As your monster works, it's stamina will go down and it will eventually need to rest. Do not over work your monster as it will get upset and get sick or hate you. Your monster will have a special var called trainer love, which you cannot check up on a normal basis. Instead you'll have to see a special NPC once and awhile to see how much your monster loves you. But it won't take rocket science to tell if the monster hates you or not, it will simply begin refusing to do anything you ask.

That is the first aspect of the game. The second aspect is the life management aspect. As the trainer of the monster, you will be responsible for feeding the monster and taking care of it. And of course that means you'll need money. Not only will there be a farming aspect where you can grow, sell and use your own crops, but you can also raise other animals to make produce such as meat, milk and normal eggs. Players will also be able to go out and collect natural resources such as berries, stones or wood to make special things. Players will be responsible for building their own home and their own furniture.

The building aspect will be limited, at least at first. One thing I really dislike is how players seem to go freaking crazy with the building, blocking off entire plots of land at a time. When a player first starts, they don't have a permit to build anything and will have to get one from the Mayor of the town. Once that happens, they can start to build their home or whatever, but they will be limited to what they can do. Not only will the players have stamina that will limit the amount of actions they do, but they will also be required to have certain resources, like wood for wooden floors. As if that wasn't enough, players are only permitted to build a certain number of objects at one time. As their trainer rank goes up, so will their building allotment. There will be empty lots right in the town for building, but players will have to BUY the rights to those lots before they can build in them.

The game is in the very beginning stages of process. Right now a player can find 1 kind of egg, hatch it and that is it. The monsters have no commands yet and they do nothing. I am looking for suggestions though. Do you think players should be able to have more then one monster at a time? Perhaps they can have multiple monsters, but can only command one at a time? And do you think players should be able to just battle other players whenever they want? Or should they have to do like a challenge/dual kind of thing?
Idea sounds kinda cool so far. I was just about suggest buying lots and/or restricting where they can build stuff, but I see you got that covered. For the monsters part, I probably would allow them to have multipel monsters but only control one at a time(especially if they don't evolve or something like that). As for PVP, maybe a challenge system but both has its disadvantages:

Battle any time - random killing, alot of player complaining; "strong" survive

Challenges - challenge spammers, "belligerent" players bored

Maybe you could do something like give them an option of which they'd prefer. It might not exactly alleviate the disadvantages but can kinda appeal to both types of players..
In response to Mecha Destroyer JD
It does sound good. Some ideas:
Allow each player to buy only one plot, but have dead space around it. The player can then buy up the "dead space" as they get money. This way, there will be a maximum plot size and no one will be hogging space.

As to PvP, why not have zones. Make a challenge arena where you can go to challenge others. Then you can have a Wild zone, where if you go out there, you are fair game.

As to what the monsters do, what will their stats be? How will they fight? The different things you have them do should build up thier different stats individually. Say, having your monster till your fields will build up Stamina, having them crush rocks build strength, having them chase away vermin could build speed/dexterity, etc...
In response to Jik
Sounds like Digimon world.... with a monster rancher name! Lol but I love the digimon games almost as much as the monster ranchers games. Maybe make a breeding system where you can breed your eggs before they hatch to make stronger monsters.