Although I haven't built a heavyweight I still have some ideas about this.
I feel that robots have become too reliable so I is hard to beat them with an electrical/mechanical fault, armour has also become too strong to make it possible to attack other robots and get through the armour especially without spinning weapons.
There could be a limit on armour thickness but I doubt that will happen, I think there shouldn't been an out of the arena instead robots could be thrown around until they break - but again this is unlikely to happen due to reliable technology.
It is also hard to make fights interesting without powerful spinning weapons but I see why these cant be used.

Watching some powerful flippers is interesting and impressive but when all the robots are flippers it gets boring, but there are very few other effective weapons and the few which are effective tend to be expensive. I don't know how heavyweight competitions work so this most probably already happens but, I think it is important to make sure everyone gets the same amount of fights no matter how well they do to make the events worth going even if your robot won't win.
You could also have separate trophies/prizes for different categories of robots so an axe bot still gets first place axe bot (or non flipper) even if it got beaten by a flipper.