You can motorize you set with a XL motor & a M (or servo) motor for the direction, and also a remote control + IR reciever to control it. But the PF parts are quite expensive and I find that motorize a model is not very interesting. Here a video that can help you to motorize the set :
As you mentionned, you can add an air tank to improve the pneumatic system, but it is also quite expensive. You can also imrove the design, but you could need orage part, that are not very varied.
But finally, the only limit is your immagination !