Learn how to convert model excavators, loading shovels, dump trucks into fully function radio controlled construction equipment.
Diecast construction vehicles can be some of the most suitable models for rc conversion. The reason being that firstly they are usually quite large vehicles which makes it easier to hide the rc control circuitry and motors. Secondly a lot of these vehicles have a pivot for steering such as a loading shovel (wheel loader I think amercans call them) or they have some kind of track both of which are slightly easier than finding gears for a steerng rack assuming your model even came with one.
As with your homemade rc tractors you can also add as much control as you like to your rc construction equipments. Servos can be used to easily move the arms and buckets of your rc vehicles, alternatively if you have a big enough machine you can add real hydraulics or possibly pneumatics however this usually requires quite a large model.
You tend to need more complex controllers because of the increased number of function on the vehicle so it could be beneficial to build your own controller from scratch. Lighs can be easily added to your rc vehicles using LEDS, engine sounds and actuator sounds can be added using a microcontroller if you want it to sound more realistic.