So one thing that is kind of annoying to me is joining friends, the reason why i say that is because we're never on at the same time, and if one of us is not on the others can't play, and vice versa, and one of the things i like is progression, so here's my idea:
Give the ability for each player to create their OWN hub, and have that HUB their OWN tiers (pretty much like single player within a multiplayer enviroment)
What this would do is give people who are barely starting up with a friend give them the ability to unlock everything themself instead of mooching off / losing ability of progress
Maybe even allow single player games to load their progress onto another persons server, so if you join a friend who is tier one and your maxed out you can still make tier 4 stuff to help him or to go make your own stuff while still playing with your friend.
This would also enable the ability for friends to join in and make their own area of construction in a friends world and then you could send your friend resources etc via the newly made trains that he doesn't have access to yet.
Yes i'm thinking of this of the point of having each person with their own tiers their own progression, and it would enable the ability to work with a friend with your own tier (upgrade their stuff etc) or while your friend is learning the game go off and make your own little base and you'll be able to fly over and help them when needed :)