So taking the TfL Central London Bus Map as an example because it makes an excellent street map. You make a simple Z fold in the A4 landscape page to get it to fit your A5 organiser:
Shown opened out.
So if you think, ok but bigger still would be better... A3 is also possible in an A5 organiser! A3 is 420mm by 210mm. So the folding is a little more tricky, but not impossible.
Again using the Transport for London (TfL) bus map. Fold up at 210 mm from the top along the whole length of the page. Then a diagonal fold in the bottom left hand corner so it doesn't fowl the rings.
Then we Z fold it at 147 mm and back again like the A4 sheet.
And here it is fully folded.
The TfL site also has Tube and Rail maps, plenty of different formats to print out and use as you wish, very good if you are visiting the Filofax shop in London... For other countries do a Google image search and look for Hi-Res images of maps, you will be surprised what is out there.
So you have no reason not to include those standard office documents, telephone lists etc in your organiser.
Using similar folding techniques it's possible to use bigger pages in a personal size organiser as well, just look at the year planners from Filofax.