I am talking really simple. Basically a hull just capable of accepting StanFlex modules. Something akin to a stretched Arafura class. Really just the OCV that the navy was originally going to get.
That is something which, in order to be useful, will not be simple, inexpensive, or quick to bring into service.
I could be mistaken, but IIRC the cost of sensors, electronics and the combat management system used aboard modern warships tend to account for somewhere between a third and half the initial cost of a modern, reasonably advanced warship.
Further, while I do like the StanFlex module concept, to date only the Danes have adopted it, or produced modules for it. This means that either Australia would need to license StanFlex designs for production, or develop an indigenous equivalent. Further, Australia would also need to license the systems architecture which enables the modules to be 'plugged into' sockets to work with the CMS used by the Danes, or adapt the systems architecture to work with the CMS used by the RAN, which seems to have largely settled on 9LV. If that sort of work is not done, then one could load all the weapons one could want onto a ship, but with no tie-in between the sensors, computers, and weaponry, hostiles could not be engaged.
While I like the concept of the RAN having smaller vessels which can be rapidly 'upgunned' from a pool of flexible weapon modules, to date the RAN has not, to my knowledge, committed to doing so, or taken steps which would enable this to take place. At this point, while the RAN could embark upon such a development programme, it would likely take a number of years before units start to be built, never mind begin entering service. Also as others have pointed out, there would be problems due to budgeting and manpower, and likely available build space too.