Best gameplay-changing mods?
Hi there. Firstly I believe this post is in the wrong subforum, it should be
here.
As for your question,
RSO is the most obvious mod that matches all your criteria. It changes the game in a subtle way but still manages to affect gameplay significantly.
If Bob's and Angel's are too much for you (don't worry, they were for me too), I'd suggest you try
AAI Industries. I've been using it on my most recent playthrough and it manages to give the game a whole new feel while leaving the core gameplay basically untouched. It makes tech progression feel a lot more natural than vanilla IMO, but be warned it ramps up the difficulty quite a bit. The biggest downside to it is that most of the changes only significantly affect the early game, so the late game can still feel pretty vanilla. Also, it significantly increases the resources you need to make a lot of the basic items, and as such can quickly unbalance other mods.
Besides that, the other AAI mods (programmable vehicles/structures) are definitely worth checking out, and can add a new dimension to the vanilla gameplay without feeling overbearing. Then there's a whole boatload of mods that add singular, contained additions to the gameplay, such as
Avatars,
Aircraft,
Bio Industries, and many more.
Lastly, there are a plethora of quality of life mods that are worth checking out and are a fundamental part of the game IMO (I won't list them here, since I don't think QoL mods meet your set criteria). My advice is to hit the
Mod Portal and just go crazy. You're bound to find something you like.