I have been playing since 2016 with thousands of hours in Realism Overhaul. The stock game was too cartoony with the 9v and AA batteries on your probes and those cartoon textures. Once I got good at using Mechjeb for precise planning and maneuvers I never looked back.
The most important thing for me is design tools. I need procedural parts, wings, with curves, straight lines and hard edges in a variety of geometries and offsets. Even the current iterations of procedural parts limit the NON-symetrical shapes to a minimum ie. Radial Boosters in the Real Tanks. The small number of goofy parts in the stock game would have lost my attention long ago. Realism overhaul and RP-1 are the things that keep me playing.
If you want staying power and long term interest in the new game it needs these things or it will flop fast.