For a thread of the conversation on the same topic I had in Mastodon you could follow this link, surely you will find the activity and the links very useful.

I'm planning to do it genuinely hard sci-fi, so it will have to deal with rather complicated things like those 5.4 Sv of surface radiation (see Colonization of Europa - Environment).

As for the player characters I'm planning to have a crew of scientists and, probably, some military types.

Yes, the sub will need to be very strong and prepared to deal with depths of 100 km Europa - Subsurface ocean.

You can also ask @[email protected] , he wrote about medieval demographics years ago

Perhaps this calculator can help you: donjon; Medieval Demographics Calculator

The referee should think about the scalability of the adventure / campaign and in level-based games this means how many levels can be obtained. Let's see an example: Imagine a scale of 10 levels, being 10th the maximum available. The monsters and enemies should have a comparable Hit-Dice scale with the final enemy having 10 HD (or 11 or 12 if you want something a bit more powerful).

Interesting, there could be traits or backgrounds related to a given origin or birth place for example (something that D&D 5 and other games already do).

A poor game session could be the result of many things (not necessarily related to the game), like the mood of the referee or the players at the moment of playing or if the referee have had enough time to do a proper preparation.

I completely agree with @[email protected] .

I should add that if it's their first time better use an easy ruleset. In this case I would recommend this: Vieja Escuela: the role-playing game | VIEJA ESCUELA

Welcome aboard and into the rabbit hole 😉

These are some things I'm working on (in varied degrees of completion):

• A Cthulhu Mythos campaign plunging the players into the Taiping Rebellion
• Studying how to make a version of Eclipse Phase using M-Space (based on the Mythras roleplaying game)
• Toying with some ideas using Mongoose Traveller 2nd edition and 2300AD
• Toying with some OSR concepts using Axis Mundi by 77 Mundos

It could depend on which game systems are more famous in your country. Games like Call of Cthulhu and Runequest , both using BRP (a percentile system) were very famous and played in Spain for many years in direct competition of Dungeons & Dragons.