Well, I suppose I probably ought to weigh in at this point and I won't get another chance before the thread closes... although, I have to say that I don't think there is much more to be said at this point. All reasoning for most type combinations seems to boil down to personal preference - I have come to the conclusion that there is no objective way of comparing any of the typings proposed. Hopefully once a typing is finally chosen we can get a little more on-track and focused... I'm quite disappointed with some of the reasoning used in this thread and prior, my own included.
Fire
Poison
Bug / Psychic
Fire / Poison
Ice / Rock
Ice / Steel
I still support
Bug / Psychic and
Ice / Steel, because a) both of them are pretty horrible both defensively and offensively and b) both of them have some sort of viable, currently unfilled niche that they could potentially fill, as a direct result of their typing. For Ice/Steel, this could either take the form of a properly viable defensive Ice-type, in a similar vein to that of Ferrothorn, or an Ice-type attacker with usable resistances, or indeed as a possible universal dragon counter, something that I am very intrigued by indeed. I am perhaps less happy with Bug/Psychic but I still think that it opens up some interesting avenues - and more importantly, I think it's the most notable offensive typing on the board at the moment, simply because it is both offensively and defensively lacking, even between its two types, meaning that it's less of a straightforward powerhouse than one of the Fire-types (which would, I think, typically only use their Fire STAB moves), yet perhaps wouldn't necessitate a novelty ability or powerful stats to make it usable, unlike Ice/Rock, and has more exploitable defensive weaknesses than Poison. Perhaps I am being a little hypocritical in liking Bug/Psychic while disliking Ice/Rock and mono-Poison when their niches will, in all likelihood, be largely the same - however, I think that for me, Bug/Psychic is the most balanced of all the types and largely the least competent, without being overtly so.
With regard to Ice/Rock and Fire/Poison, I think I have already touched on why I don't like them. Fire/Poison has terrifically strong offensive potential and has Heatran-esque resistances and weaknesses, though I don't like how many people seem to be lauding it as a catch-all answer to Scizor, when such Pokemon are hardly uncommon - Zapdos comes to mind, for a start. Fire/Poison is largely a reasonably good typing regardless - the potential for Fire/Poison as a Toxic Spikes absorber that doubles as a strong attacker is huge, but then outside of this niche I don't really see much potential for it. Ice/Rock I see no potential for outside of braining whatever happens to switch into it and then running away screaming. This may be some peoples' cup of tea, and more power to them. But I'd prefer a more elegant answer to this CAP's concept.
Fire and Poison I have already made my position clear on. Fire is an excellent offensive and defensive typing that suffers through its weakness to Stealth Rock, and hence I feel is limited to an offensive role. Poison is hardly a bad type either, and any limitations in its coverage are easily solvable by stats and movepool. I feel as though these two options are far too easy, and hence that they are a waste of this CAP's potential. However, I would admit that Fire is a better proposition than Electric/Fire.
That's all I have to say this time. Onwards and upwards.