Heh, what's this? I worked in FX at one point in time and found the whole thing very intriguing and inspiring. Maybe you can elaborate? There could be an overlap of interests.
But always using the FX approach to trading: Buy/sell, base/terms, three-letter-code currencies, spot/fwd and swap.
If liquidity emerged for more markets from user requests on the site, you could add them later. To be honest, it's such an obvious idea that there's either something wrong with it, or someone is doing it already, or both.
Virtually all of my current work is with that goal (logicscope.com). The group took a dirty problem - integrating dispirate banking systems - and wrote a cool abstraction that lets it work as a multiadapter. Plus support, configuration management, etc, etc. By accident, the system also turns out to be a not-entirely-elegant but much cheaper and easier-to-work-with replacement for MQ for many settings.
The idea of queueing and releasing a chain of transactions on the trading side of the transaction may be interesting but I can't see it yet. Are you referring to situations in which you do a single "trade" but with a complex series of legs? If so - this already exists in some organisations but is not widely supported by APIs or major GUI interfaces yet. Or are you thinking more of forming a position across a range of positions and then unleashing it? This could be useful, but I'm not sure who you'd target it at. The banks are likely to have internal software teams doing a mixed job of delivering this sort of fn already, and I'm not sure how you could base a startup on it. I expect there would be some hedge funds that could do this to come up with interesting situations - for example - setting up pricing queues for stocks floated on multiple exchanges and currencies and then doing automatic arbitrage based on this. Algorithmic trading isn't my speciality though, and I'm sure there are smart people doing this already :)
The STP I'm referring to relates to automatic roundtrips including request for quote > pricing engine > quote > order > confirmation > TMS but the real interesting part is when banks automatically back their orders with purchases from yet other banks, intra-bank. Exotic currencies add an additional dimension, as no immediate pricing for e.g. Mexican pesos in Norwegian kronors is available and the trade needs to be 'routed' through an intermediate currency, usually USD or EUR.
My view of the system didn't go beyond two banks at the time but in theory there could be a long chain. That way the banks would be forming a financial P2P network. In reality it's a pretty hierarchical network as not all banks are created equal, but it'd be interesting if you could come up with financial routing mechanisms that would find the cheapest route in a hypothetical more dense banking network and what you can learn from that for poor man's P2P networks.
There's a couple of problem with deliberate chaining of interbank trades in that if the market moves far you might not get your countertrade (the quote will be rejected on the trade attempt - it's more common for quotes to be non-guaranteed than for them to be guaranteed, presumably because it's easier to build and avoids the situation where hedge funds start gaming you); and - irrespective of that - prices tend to get naturally stale fairly quickly (every few seconds but it's usually inconsistent) which means that latency is a factor.