By far, the most successful modification, not to mention my overall favorite mod, would have to be a plane I built doing what we call “KIT Bashing”. Kit Bashing is where you have parts and pieces that you put together from two or more airplane kits and try to invent a plane.
What I did was take a couple of wings from an Extra 330 I had and a fuselage from my old MK Magic I pretty much ended up turning it from a mono-plane to a bi-plane. While it wasn’t that pretty to look at with mismatched color themes, it definitely had that Frankenstein feel to it. 
It did fly.
Even though it was probably the ugliest plane I had ever flown, it was actually a GREAT flying plane. I seriously had a blast with it!
Because it was now a bi-plane, the slow flight capabilities were increased a lot! The wing area was greater so it was able to float more and overall it was more draggy. But, because I had also used 3D components, it was super aerobatic! It was capable of lots of different tricks. I could hover it, harrier it, and even do roll and harrier circles.
I have kit-bashed plenty of times before and since that plane. I have seen a lot of my buddies do it over the years. Almost none of them turn out very well, but that one sure DID!