I think allowing UTVs would be popular, but it's probably not a good move. At least for stock UTVs with tall ride height and flimsy rollover protection. They're definitely tippy in stock form, and there are a distressing number of pictures online of stock UTV cages absolutely squashed flat after a flop.
The major issues I have with the current ruleset:
-Disallowing non-automotive engines. Naturally-aspirated 4 cylinder or smaller motorcycle, snowmobile, ATV/UTV engines should be acceptable and are what people choose for small lightweight buggies. It was a good first step to remove the ban on Crosskarts, Legends, etc, but the drivetrain these use is still illegal.
-The 90% height to track width rule is too restrictive. That's an awfully low overall height for a vehicle designed to be used exclusively off road, and doesn't leave a lot of room for both adequate ground clearance and adequate helmet-to-cage clearance without an uncomfortable layback seating position. Why is the rule for autocross 1:1 width to height, but 90% here?
-The roll cage requirements feel a bit ambiguous. Pointing to an appendix in the GCR for 2007 and older roll cage requirements seems odd. What would be nice is a document similiar to the one 24 Hours of Lemons uses to supplement their rules, the infamous "How to Not Fail Lemons Tech Inspection" pdf.
Slightly smaller annoyances in the rules:
-The full coverage fender requirement, as mentioned above. Requiring mudflaps is one thing, and is reasonably easy to retrofit to an existing build, but full coverage fenders on an open wheel buggy is a tall ask. Locost builders often have a hard time getting fenders to stay attached to the front of their cars, and they're not bouncing around a gravel field.
-Only tires designed for four-wheeled road-going passenger cars allowed. Even the smallest, hard to find rally tire is going to be wrong for a sub-1000lb buggy. There are lots of UTV tires available these days that are DOT rated, have a reasonable not-too-agressive AT/MT tread pattern, and are designed for the weight and side loading of 4 wheeled side by sides. There are some good DOT options designed for golf carts too. I think as long as the tire is DOT rated and the load rating on the sidewall is adequate for the vehicle, the Constructors class should have more freedom of tire choice.