Essentially, the proposals so far seem to suggest four, not necessarily exclusive, possible uses for money.
A) As an "input" type resource, to feed research, parallel to the way minerals will feed industry. Follow up things that tend to flow from this line of thinking are; money should be a focus; planets should have either generally variable aptitude for money production (similar to mineral richness, e.g. "natural tourist attractiveness".) or a fair variety of money enhancing specials; Needing to gain control of Money Producing planets would provide the map based strategic part of the research strategy.
B) As an "output" type resource, to pay for spying/diplomacy type stuff. I.e. to enable you to wage covert/economic warfare, to provide another arena to play the game in (in parallel to the military and industrial sides of the game). Flowing from this; we really need to develop a pretty complex diplomacy/spying side to the game to make this viable, as we'd need to make players actually be desperate enough to get their hands on some money that they'd be willing to trade techs/alliances etc. for it, so It'd have to be useful for some pretty good stuff.
C) As a way of paying maintenance costs, possibly on ships; This essentially makes available money supply a cap on economy/military size. (Personally I'm not so keen on this, since I'd have thought having maintainance take industrial output would do this job just as well, but it seems like more people like the idea of upkeep taking money, so who am I to argue. I suppose it does serve to give money more value).
Oh, I've just had a thought. If money were needed to feed research, what if money was required alongside (production or minerals(i.e. "materials")) to maintain a fleet, with large low-tech ships needing more (minerals or industry), and higher-tech ships requiring more money. This would mean that low-tech industrial giants are eventually hamstrung by maintenance, but so are high-tech players, at about the same relative effectivenes in their strategie, thus avoiding the "if the tech race stays alive till halfway they'll be unstoppable" maxim, and making it a straighter fight all through the game. Actually this would work even if money were a spying resource rather than feeding research, it'd just be a bit less aesthetically pleasing.
D) Needing it for "social" reasons, i.e. to reduce unrest on your planets, or to increase (nebuluous) social ratings (health/education etc.). The difficulty I see here is that we'd need something that was a more relevant gameplay mechaninc than unrest was in Moo3 or Civ, or players will just see it as an annoyance. Perhaps we could incorporate this idea into the spying/diplomacy part, and use money to keep your empire together and working (It might be in continual danger of splitting apart for various reasons, unrest, independancy movements, fear, rebellion of newly conquered worlds.), but allow opponents to use money to encourage that splintering. Then it could become a move and counter move part of the game, looking for opportunites to target vulnerable parts of an enemies empire, raiding their borders to scare the populace, funding independance movements, supporting piracy to increase unrest etc... Might be fun?
I'm generally against having money only echo the size of your population, since that ties anything you do with money inot being essentially based on your empire size, which limits a player's strategic options, i.e. they can't be forced to choose/trade off between following whatever strategy money supports (research/spying/fleetsize etc.), or following a conquest/expansion strategy, since they must do the latter to increase their money supply. So I'd support money as a focus.
With money as a focus, how will income work? ie, will a world with farming as a primary and secondary focus provide any income for the empire? If so, where does that money come from and how does it get calculated?
If we went with money as a focus, I presume we'd treat it in the same way as all the other focii, so how would we answer the equivalent: "With food as a focus, how will food supply work? ie, will a world with money as a primary and secondary focus provide any food for the empire? If so, where does that food come from and how does it get calculated?". I.e. I'd assume the double-farming world wouldn't produce any money for the empire, since it's busy doing other stuff (or would produce minimal money if the equivalent farming world would produce minimal food, I haven't checked the Req. Docs. I'd personaly prefer it if you produced nothing of anything else on a "double focused" world.)
I'd also like to advise caution before heading into tax rates, particularly where there's a danger we'd end up with multiple interacting levels of taxes (e.g. imperial,system,planet). The corollarly to the "steal what works" rule is "stear clear of what didn't" and MOO3 showed that it's easy to turn that into a stinking mess if we're not careful, so let's make life easy for ourselves if we can.
I wont get into realism
Good job, because this is a 'realism' argument that is specifically banned by Aq. (I should admit that's mostly because I (and others, but I won't name names) get real prickly about it because we see the realism argument pointing in exactly the opposite way on this point..., so it tends to derail the argument into petty squabbles.)