Quite a few people have tcase gear changes, and most people would agree that it can be a double edged sword leaving gaps in gearing between first high and top low. The people with single cases are likely to be the ones that have the more moderate gear ratios (ie <3.8:1).
It would seem more feasable to have a difference between Tryer and Challenge of not weather they have gears or twin cases, but on the overall maximum gearing difference from transmission output to tcase output, in other words (say) <3.8:1 or>3.8:1.
There are vehicles around that have 4:1 low ratio as standard (eg. Jeep Rubicon) and these would be at an unfair advantage in the Tryer and Standard classes.
Tryers class: spiring overs allowed, but no additional links...Does this include an trac bar? I would think for most vehicles that would be an on-road safety issue.
For Standard class a "+2 inch" limit on the tyre size as alowed by the new NCOP. The first thing ppl do is get bigger tyres. You really don't want to limit this class to "straight off the showroom floor" with absolutely no "common" mods allowed.
Not being critical of the idea, (can't bloody wait to see them

) but a bit of fine tuning of the rules may be needed.
My 2c worth(less?)....
George Carlin, an American Comedian said; "Think of how stupid the average person is, and realise that half of them are stupider than that".