Well, the really cool variants are cool because they are the unlikely ones!
There are lots of modifiers for diplomatic checks with any given country. Obviously the decisive thing is which side has more points worth of diplomats there, but there are also others. Speaking in boardgame terms (and I think CSS, you know this), number of diplomats determine the column used on the resolution table, while other modifiers modify the dice roll.
Most of those roll modifiers, you can't influence. You either have them or not. Like, capture of enemy cities, Belgrade for example.
There are a few which can be given attention to, and these are the national one. For example, no Austrian diplomat to Romania or Italy, if possible, and let the Russians handle Entente diplomacy in Bulgaria, if possible.
So, the unlikely results: I would not waste resources on Japan as Central Powers. It is an extremely hard task to get them off their British friends, and there is an event which simply makes them declare war on Germany regardless of diplomacy levels. So altough it would be extremely cool to have them, I would rather see them in a mod for the game, than to waste my way too precious diplomats on them.
Italy: your best bet as Centrals for them, is to get them in the beginning. Choose the Option "Diplomatic Poker" by Germany, during the Warplans phase, and you will have a dice roll determining if Italy will fulfill what is their legal binding by treaty to join Germany (since with the Poker option, Germany waits for declarations of war on her). Throw in the handing over of Trentin by Austria-Hungary, which is also an Option, and you have a very likely result of Italy joining you immediately.
But, assuming historical or near-historical warplans, the Italians will, by all likeliness, stay neutral initially, and be drifting toward the Allies, with unpredictable speed.
Italy will be a favourite target for AI diplomats, and as a matter of fact, should be a favourite target for any human player, Allieds or Centrals. As Entente, you just can't have them soon enough, and as Centrals, you need to delay them.