Main question is what are some tips to incorporate or improve/increase the hip rotation on the forehand (or groundstrokes in general).
One foot drill helps with this. I've introduced this a few times, but here's an unique variation.
For a rightie FH, put your left foot on a stool (or low chair), while keeping your right foot on the ground. Practice swinging a few times, always making sure you swing through an imagined contact point. Then, swing normally with both feet on ground.
You'll notice that your weight transfer is better, and that your pelvis turns more forcefully. This teaches you to move your weight forward through the shot, which is what you are really trying to accomplish with the hips.