I reckon the Canberras are too big for our needs.
Three or four smaller vessels like the Garibaldi or the San Giorgio
I would respectfully disagree. Although my knowledge on the operation of naval vessels is very limited, there is the saying that steel is cheap and air is free. By only building 2 bigger ships even if you don't use them at full capacity all the time you have the option of increasing your 'on the beach' presence without increasing the number of ships. Also by only building two - although the per ship costs for the larger vessels would be higher - to reach the same capability with smaller ships would require the purchase of more radars and other systems plus the overall crew requirement I imagine would be roughly similar.
As for what next with the IFAN, perhaps doing different ships through the years? Opening it up to encompass the whole 20th Century would give you a whole boatload of ships to draw (depending on how comfortable you are with ships of different eras). Otherwise maybe do some ships of allied navies? That could be interesting...
Also I think an aircraft carrier up to the size of an Invincible class wouldn't be implausible considering we were close to purchasing HMS Invincible herself at one stage