I have a cosmos where FTL travel is via an alternate dimension. The problem is when you return to our universe, you find yourself at the right destination, but a completely different parallel world from where you started.You can't control which parallel universe you end up in, so you can never go home and never go back. Technological advancement is stifled because no one wants to spend a lot of research money inventing stuff that may have already been invented elsewhere. Currency is worthless and you can only bring valuables you can keep on your ship as you journey. What easily transportable valuables would people want retain between various Earths or colony worlds and be paid in?
- Assume most ships are large and cheaply made, intended to establish colonies and dump excess population. Alternatively, some are self-financed by rich starting crew members who live permanently aboard. Mineral resources are an obvious choice, but our crew has a smaller ship originally built to survey new stars and is small.
- Patent laws would be unenforceable, since the travellers would never go back to where the things were invented. Most places would have similar technology after the initial flurry of interplanetary cultural exchange. I was already assuming geological surveys wouldn't vary much and the crew could go back to a place over and over selling the same data until other surveyors saturated the market.
- Panspermia: There is little alien life, and what there is would be simple and much like primitive Earth life. On the upside. worlds are not severely alien, and don't require extensive terraforming.
- Close similarity. Most worlds are similar in basic physics and at least related historically. Sure, here the Nazis won WWII, and here the soviets ruled the world while in another the only differences are cosmetic - you can meet yourself and swap stories about your childhood, never even figuring out the difference.