Bloomberg does have a place on most brokerage and trading firms and while some use it, it normally is not the main platform for trading. As TheBigShort says, they have a bit of a monopoly on the messaging. The order entry is not fast (although you can use EMSX if you wish) relative to what is out there and of course the price is very dear. You certainly won't find it easy to trade global asset classes and multiple products on one screen with fast order entry for example.
The IB TWS offers entry for all product classes and with hot keys, basket trader, algos and more, does the job and yes, the TWS does sit on many institutional desks. But there is no one size fits all so if you believe you can design something better, I'd say go for it but keep in mind your design would most likely fit your needs and not necessarily the masses. For example at many institutions you have one person or team trading one product or sector. Each team will have their own needs and concerns. There is a plethora of really good front ends in the market - many of which have designed for TWS via our API. The key though is that many of them are tailor made for specific needs or products. We have serious "power users" who connect via CTCI others, who do it all from TWS classic, others Mosaic and others who via basic excel APIs to others using 3rd party front ends or sophisticated APIs they wrote on their own.
Also keep in mind there is a lot more to order entry. I for one believe the real power is the execution quality and work going on behind the scenes (taxes, corporate actions, data management, database management, stock loan, etc, etc). Depending on the end users needs, these important items need to be addressed within the design of the platform. In short, while I have little doubt you and your team can design a great front end, I highly doubt you'll come up with the holy grail.
