Perhaps a way that reduces xp gained from any one source over time and have it slowly return... Like how solar would wax/wane during day/night cycles. This would force players to actually be doing things, building gives xp but slows after a bit, then you could mine resources for xp, which would slow down, but give you building xp back, or asteroids to give you xp, then return other sources.
This both punishes players for possible xp exploits, but at the same time rewards them for literally just playing the game properly ...
Have it gradually return over time, AND with actions. This could be over 1 hour realistically as regular play would push that closer to 2-10 minutes if they were to build/mine/spaceship/destroy/etc quickly enough...
This feasible?
It's interesting, if not perhaps feasible as where the player is loaded is where the XP is gained. If the ship/planet/etc is not loaded - ie the player is not there, there is no xp to be gained.
However, some sort of fatigue meter that increase when gaining xp from one source (reducing XP gain), and reduces when gaining it from another source seems totally possible and is pretty much the same thing. Whilst I don't want to put bandaids on certain issues, this feels complex enough to get other elements in the mod first - fleshing out the trees that got split etc.
This would really play well in to the XP tree I have in mind and could start to have an incremental feel to it - xp starts slow, start unlocking xp fatigue reduction or go for short term gains for the factory. This, in my mind, all leans more towards dynamic/increasing point costs for some/all skills to balance it out, which I am not averse to, but it could be a later down the line job when all the skills I want or are suggested are in and it can be balanced as a whole.
Great idea, tonnes of potential, love the balancing play and rewarding efficient xp gaining styles. It would always be an option, to follow the play it your own way ethos. Dynamic/Increasing - the more skills you have the more it costs: "Hard" Mode, or as it is now, flat skill cost.
Overall: It's on the list, but there is a fair bit of work before it. I set xp limit to 3000 and it still got reduced so quickly, they'll be a tonne of fine tuning that - again the xp skill coming before 2.0 will hopefully over some balance (and the options adjusted)