Out of complete ignorance I ask this question. Ignore zoning and permitting for a minute, where could new resorts be added that have enough snow? Having driven through Utah in the winter it is mostly a desert area with concentrated snow in what appears to be a pretty localized area of the Wasatch near SLC. The Wasatch is pretty narrow East to West, unlike the Rockies in Colorado.
The only spots that could really compete at the level of Snowbird and Alta (snowfall, snow preservation, size, and terrain quality) are in federally-designated wilderness areas. The Wasatch core could theoretically support 6 additional areas more or less competitive with Solitude... one at the top of Millcreek Canyon, two west of Solitude, one north of Brighton, one west of Snowbird, and one between Alta and Brighton/Solitude. All but Millcreek would add congestion to the already-burdened LCC and BCC roads. All receive significant summer recreation, and all but Millcreek are already at least semi-popular sidecountry.
To the west, the Oquirrh Mountains probably represent the best opportunity to really get something permitted. It's a very steep range, so there's limited novice/intermediate terrain available and real estate development poses extra challenges, but the right developments there could definitely lure some of the advanced skiers from the Wasatch.
To the north, theoretically there could be a string of resorts primarily facing east or west from I-80 all the way up to Nordic Valley and beyond. The two best spots are probably Snowbasin and Wasatch Peaks Ranch, but there's plenty of room for more. PowMow has tons of terrain dedicated to catskiing, and on beyond that there's a lot more similar terrain.
To the south, there's some very promising terrain just outside Provo, the old Heritage Mountain/Seven Peaks proposal. And up American Fork canyon there's a a ton of terrain with lots of potential south and east of Snowbird. But no good access to the area.
So yeah, there is theoretically room for a whole lot more ski terrain in the Salt Lake area. Realistically, I think one or two new ski areas in the Oquirrhs and some sort of Nordic Valley expansion are possible.