AI successor vs successor still working out well, with meaningful, but balanced combat, and costly for each kingdom, which represents that struggle quite well. Though in this version, when I played the Antigonids, I was much less likely to lose Syria to the other 2 even when playing carelessly on purpose. This may or may not be desirable, depending on how difficult you intend the Antigonid start to be. I believe I'd advocate for a stronger Seleucid Empire against me at start, because Parthia really does most of the job for me collapsing them, making me only really need to focus against the Ptolemaic Kingdom.
Turns out that Antigonids owning Athens from start is not ahistorical. Maybe their heavy spearmen could be moved there, in the hopes they take Pella. This would also, historically so, weaken their position in the Levant. Just an idea to get the Antigonids to act historically.
http://upload.wikimedia.org/wikipedia/commons/f/f3/Diadoch.png
Update: Playing as Seleucids this time, and it's quite challenging, in very fun way that forces you to think strategically. UHV is much harder, but I don't think it is impossible. Their UU is only advantageous when on the offense, which is a nice touch.
Maybe Rome's free units on war declaration against its neighbours should only take effect after 200 - 180 BC (time of the 2nd Punic War), to give the AI, and the human player, some time to take in the Hellenistic setting of the 3rd century BC before the Roman takeover. I don't think this limitation would hinder Rome's progress. Currently, they get to Mesopotamia before the Parthians even spawn!