Best mods... That are not parts?

[Hooligan Labs]

Have modders experimented with scenarios or significant changes to gameplay?

[astropapi1]

Are you talking about plugins?

[Hooligan Labs]

partly, but most plugins are made just to support parts.

[EndlessWaves]

Yeah, there are few now. For example deadly re-entry and kerbal alarm clock. Arguably some of the mods with parts like Kerbal Engineer and Protractor would qualify as well as the parts do nothing except launch windows.

[ummwut]

The Subassembly plugin is awesome, definitely changed how I constructed ships, and how quickly I was able to do so.

[nothke]

partly, but most plugins are made just to support parts.

Some are not made to support parts, but parts are made to support plugins =) For example mechjeb. It's just one part (as of 2.0), but it is practically just used to initiate the plugin. That also counts for example for BOSS or some pieces of Lazor system.

Out of "global mods" I have only used deadly reentry and it really changes the game a lot, you need to plan and test much more and be very cautious on reentry angles and speed. But I haven't seen any other mod that "significantly changes the gameplay" as you said.

PS: Subassembly loader is a great tool! But it's a tool, it doesn't change gameplay

[knossos]

Out of "global mods" I have only used deadly reentry and it really changes the game a lot, you need to plan and test much more and be very cautious on reentry angles and speed. But I haven't seen any other mod that "significantly changes the gameplay" as you said.

RemoteTech significantly changes the gameplay, and so does the kethane mod. RemoteTech requires you to establish and maintain an array of communication satellites for unmanned craft while Kethane allows you to mine resources from planets.