Some of this sounds like what I would like to do with my Open Transactions server. It is currently just in alpha testing, see
https://bitcointalksearch.org/topic/open-transactions-server-assetbondcommoditycryptocoindeedsharestock-exch-53329However, I would prefer not to deal with fiat.
But that is where the existing exchanges come in. They deal with fiat currencies.
They also deal with all the tiny little "I want to buy 0.01 BTC" kind of stuff, which I would also prefer not to bother with.
I have set up my currency contracts so far to treat coins as whole numbers, and I hope that I will not have to allow decimals since for example if you want to sell some coin or other for 2.99 of some other coin you could offer 100 of the first coin for 299 of the second coin on the units-of-100-coins market of the first coin. I am hoping that using whole numbers might help cut down on the number of frivolously tiny trades.
Open Transactions uses quite secure communications, but that makes it not all that handy for web-based use, as the kind of people who insist on using a website tend to also insist on using pathetically easy to guess passwords and stuff like that. So basically this would not be targeting the casual user who deals in such trivial amounts that they basically do not bother with much security. Such people are just going to give the service a bad name anyway by constantly managing to get ripped off, hacked, trojan'd etc etc anyway. So we'd hopefully be targetting serious people who want the good security.
Probably instead of even having MtGox USD at all, I'd only have MtGox BTC, and let people do the conversion to fiat over at the fiat-to-coins exchange. I'd try to focus on *coins and on stocks, shares, bonds type of things.
By not dealing in fiat I would presumably be able to have much better liquidity across the various exchanges that do handle fiat.
Also, maybe I could have tokens that represent BTC actually held at third party exchanges, so one could trade that without having to actually move the BTC from exchange to exchange. The added risk though compared to tokens representing BTC I actually have in my own secure wallet might make the tokens representing coins held at third party exchanges trade at a different price than those representing coins I have direct control of?
-MarkM-