I would suggest a system that I'm sure the Devs have already discussed. Options for teirs is a good one, but in addition to this: have a main portal, one that goes in a base, then lesser portals that all connect to this one via the named system currently being used, the lesser portals would have tiers to allow differing levels of material to pass through. Even have a first tier that only lesser material can be put through but players couldn't travel through. So you can still fast travel between bases, but stay true to adventuring.