"Work as if you live in the early days of a better nation"
So, I've only got a couple of games under my belt but already the wheels in my head are aturnin' and I've come up with a few proposals for some new rules.
These ideas primarily apply towards balancing trade against war, as well as dealing with the pesky diplomat.
Okay, so here they are:
1. Trade values for land areas should be rounded up, not down.
2. The Mediterranean areas should have their values inverted. So, they are worth 8 victory points for trade control and 4 for military/political control.
3. Trade cubes are only plundered if they match the conquered political control marker. Once a player "crosses over" to another culture, they cubes cannot be swept away by general conquest and will probably remain there unless converted later.
4. The diplomatic action always takes two trade cubes up front. Therefore if a diplomat wants to try to convert another player's region, they must spend two trade goods, plus any good needed to modify the die roll.
5. There have been some complaints about a runaway leader problem, I have not personally experienced this, however, I would be interested in implementing a rule whereby, if one player ever moves so far ahead, they are granted victory at the end of the turn (say 15 points more than the next highest player). This would create an interesting alternative victory condition but would require careful balancing.
I'd be very interested to hear what other, more experienced, players think of these changes.
ἄνδρα μοι ἔννεπε, μοῦσα, πολύτροπον, ὃς μάλα πολλὰ/ πλάγχθη, ἐπεὶ Τροίης ἱερὸν πτολίεθρον ἔπερσεν./...
μῆνιν ἄειδε θεὰ Πηληϊάδεω Ἀχιλῆος/ οὐλομένην, ἣ μυρί᾽ Ἀχαιοῖς ἄλγε᾽ ἔθηκε,/...
I think eliminating plunder (which will in practice be what you're doing given your suggestion) would tend to un-balance the game, not the reverse. I find thegame balance excellent.
Icould go on, but really I would just recommend you play the game as is for a while longer before tweaking it. This is one of those excellent games which take a while to really understand the strategy.