Hi Puke... I knew you liked Imperialism...
A potential problem: I suppose a good tactic would be to destroy homeworlds of those neutral empires which have been subjugated by another player. This can be done without declaring war to the human player, so that (at least for the first turn) you won't have to fight his ships (I suppose every human player will place ships/units/whatever to defend his colonies). Since the AIs aren't very good at defending themselves, this could be exploited in some way (maybe).
Also, what about gifting your own planets or colony ships to your subjugated neutral empire? I think this is ok as far as game rules go, but it's somehow weird... you'd gain more resources by having a single homeworld (or maybe not even a homeworld) and gifting everything else to your neutral colonies...