It looks like mapSize can be redefined for the Fallen Worlds scenario but I didn't put in a request to change it because the nebula-producing code is quite elaborate and I don't know how it would respond to a different-sized map. George probably knows. Given the reduction in mobility that jumpbeacons have introduced, I'm not sure how I feel about a more spaced-out map- it could be interesting, but it would have to not be so large that capital starships can never get anywhere (unless they were also given a speed boost).
Another thing that I think would be worthwhile to implement before a new game - and which could be accomplished by changing the libaries - would be to either tweak the names of planetary resource abundance traits or add icons that show the modifiers they impart. I think players would benefit from the game explicitly telling them that "minor trillum deposits" are actually "minor trillum deposits (-50%)". Additionally, the planetary biosphere traits should not have "hidden: true", since these traits impart production modifiers to organic food. (e.g., I just discovered that empyreal worlds get a food bonus.)
On the topic of production modifiers, it looks like you can define all kind of interesting traits to planets modifying the production of just about anything, including components and units. So in theory there is no impediment to having a planetary feature trait that reduces the WU (not resource or component, I think) cost of light jumpdrives or basic infantry built on a planet by 10%. You could apply this to all worlds of a specific type (e.g. give all primordial worlds a 50% malus to building every starship class) or randomly apply traits to worlds when a scenario starts (e.g. 10% of worlds created at scenario start gets one of a random selection of twenty possibly minor production modifiers to various components and units).