Just leave the diplomacy template making to professionals such as Only The Pie, MGSB or myself. At least 4/5 of the open games diplomacy games should be templates by the .

You play too few other diplomacies my friend

Ok, i have come across the same problem as you, but since you are not really competing in the game, you can regulate it very well. not every bonus has to be the same for each player. in the 30 years war scenario, england will be stronger then for example switzerland. you make the bonus territory for england larger and for switzerland smaller, in this way, if one of them breaks the rules, they will lose percentagewise the same.

Also, i thought it would be a good idea to make extra bonus territories. If someone gains a lot of land, they will, indeed, gained relatively little income. Then this player will get an extra bonus territory, which can again easily be taken away by the host. It is difficult to decide when these extra bonus territories will be rewarded, but this can be done by, for example, capturing the enemies capital city.