Since wide open plains is one of your objectives, I'd stick to the relatively drier areas east and north of the northern end of the Sacramento Valley. The coastal range is quite wet in comparison, so you won't find open valleys with no trees.
You might enjoy exploring by starting in Westwood and heading north thru the Lassen NF. You will go thru small grassy valleys surrounded by wooded hills, although nothing on the scale of Rohan. Before you get further north, walk around there a bit. Hiking thru the forest here is a different experience than in most forests. There are large Poderosa pines spread out, with usually easy walking around between them.
The climate gets drier as you go north and east. Around Eagle Lake, the area is more open with fewer trees. Further north instead of east it gets drier too. However, the open valleys won't be large grasslands but more rocky plains, culminating in a totally different landscape in Craters of the Moon and the Modoc NF by the OR border.
One thing you can do is poke around in Google Earth to see where the flat treeless areas are. Most places now have some uploaded photographs so you can see what it looks like from eye level.
For example, go to 40.580N 121.078W. That's one of the meadows I mentioned north of Westwood. It's relatively small in the scheme of things, but is otherwise what I imagine Rohan to be. It's wet enough for there to be lots of grass to support horses, like Rohan.
On the other hand, take a look at 40.903N 120.738W. That's a much bigger area, and you should be able to get a feeling of hiking around in large openess. However, it's dryer than I imagine Rohan. Further north are even larger, but also dryer flat areas. In this part of the world, the larger flat areas will be more desert than grassland.