Campaigns already have that functionality by importing and declaring stuff at the campaign level which makes it available in all sub-maps.
If you want a multiplayer map series then you will need some sort of external download which places all the assets into external folders in the WC3 install directory (aka the bypass 8 MB map limit approach). Players will need to "install" these assets before they can play the map, which greatly affects playability as online join and download will not work. Object data and triggers will still need to be duplicated for each map, however you can mirror them across maps quite easily and they are considerably smaller than assets.
Do be aware that any "series" of maps is quite an undertaking. There is a reason most map makers only ever release 1-2 maps in their entire WC3 careers. Making maps takes a lot of time that even finishing one map is often more work than most people mappers can manage. I would strongly advise designing/perfecting 1 map for a start, and consider expanding later once you have a "formula" worked out for your map series. If you are really fixated on a map series from the get go then I strongly advise recruiting a sizable team that you have close contact with and working on multiple maps at the same time in parallel.