Hello and welcome to RTA !
You will find Lot's of info throughout the forums and in the road trip planning pages in the tool bars above. You say the lenght of trip is undecided depending on suggestions, well to be honest it could take a lifetime to visit the thousands of possibilities. What's worth it and what's not, really comes down to your own personal preferences. If time and money were no object, they would all be worth it in my opinion and as well as the main sites there is always plenty in between.
From Yellowstone to Vegas you have many options, including some great driving roads and National parks in Southern Utah like Arches, Canyonlands, Capitol Reef, Zion and Bryce canyon. As you took a Heli tour to the Grand canyon it is likely that you only saw the West rim outside of the National park. If that is the case I would recommend revisiting inside the National park area, being the north or south rim. Lake Powell, Monument valley are just another couple of places you could consider visiting. Whats worth it, how far you want to venture and how much time you want to spend, are ultimately down to you. Once you have worked on it some more I am sure we can help fill in the blanks and make more meaningful suggestions.
Enjoy the planning.
Basically, a lot like Michael was posting as I was still typing !