It is still fun, to try out new rockets, to optimize the LV's to see, on how less stages you get to orbit. And now, with these easy moddable addon parts, it's even more fun.
But I experienced a bug on an addon, but I don't know if there's something wrong with the addon, or if it's in the game's core:
I used this launch escape tower found
here. It's quite useful. One safety feature that was missing still the beginning, but it happens that on stage separation the chute at the top deploys even if it's positioned at the last stage.
Some other thing felt into my mind:
Harvester, you said that you want to implement some tech tree including R&D right? Does this also includes to research for UI features like speed indicators, altimeter, the nav ball, and later even more (e.g. some orbit MFD)? This would be interesting.