A 96x128 map.

two teams, and there are 3 people on each team. Each player gets to chose a hero. For each team, there will be a base.

In random spots in the map, there will be little outposts that you must fight to command.

For team 1 and team two will be fighting each other to command these little outposts in random spots in the map.

A team wins by capturing the other teams base. It wont be easy to do it though, since it has defenses owned by the computer.

I was thinking about making this, but does anyone think this is a good idea at all?

This is called an AoS type of map. Popular maps of this type include DoTA, Custom Hero Wars, and Temple Siege.

Although it's nothing original, you should definitely make whatever kind of map you enjoy making. This is a good idea if you have some cool Heroes in mind already.

I'd only identify it as an AoS if it involves a tug-o-war type of struggle between two individual computers. If you think the map is a good idea and you would enjoy it, then you should make it.