I work at a bank, in the software architecture department. My answer is somehow related to what you say I think.
They manage a concept called Time to Market. Is the time they take to implement new functions to their system. If they want to release a new product (a new type of account for example) it requires modifications in the software.
Before the whole SOA thing, they had separated programs and had to replicate data between them. Now they are chaging that, going into a service oriented approach to the architecture.
I think what you want somehow related to that, to SOA. Probably it is the best approach.
So you can build like the base of your whole platform, a place that manages authentication (your own "passport") and then start building applications on top of it, They would communicate through services (not necesarily web services but interfaces).
I hope this helped
