Not to worry, I'm actually very interested in what you're doing. It's a bit of a hobby for me too, but I just doodle on scrap paper. I have no idea how to navigate SketchUp, but did down load it a while ago.
An increase in gradient from lower to upper bowls is done so that fans higher up are closer to the field than they would be if the grade was kept the same, right? I do like that feature, but sometimes find that lower bowls have too slight of a gradient, while upper bowls are too steep. Skydome in Toronto is a good example of that. Have you been there for football/baseball?
I'm a football fan foremost, so I do look at design from the perspective of a football fan. I look at features that enhance the football experience (horseshoe or full bowls), and features that detract from it (running tracks and awkward sight lines).
Another issue that's very important from a CFL perspective is optics. The league suffers from image problems so it's very important to make a stadium look and feel like a major league stadium. How does one do that when designing a 30,000 seat stadium as opposed to a 70,000 seat stadium? There are lots of tricks.
The West Harbour proposal in Hamilton incorporates many of the design features that make it look like a big city stadium even though it's a 22,000-44,000 seat facility. Retractable stands, full bowl, over sized industrial looking trusses, and massive structures anchoring on the corners are the major features that they've made excellent use of.
Another design that CFL stadia needs to accommodate is expandable capacity. End zones are an obvious add on, but what about building stadia that was designed for easy add on of stands? The best solution I've come up with is a sunken bowl of 20,000-30,000 not including end zone seating.
If one builds the entire bowl, but only installs seating on the sides, the bowl can easily be expanded from 30,000 to say 37,000 as a horseshoe, or 44,000 as a full bowl. Concourses at each end add awesome viewing platforms for fans circulating around the building.
20, 30, 40, 50 years down the road a sunken bowl also allows for the easy expansion of the stadium if enough room is left on all sides. Simply build a 15,000 seat grand stand on one side. If a stadium is designed with this in mind from the beginning, a 2nd tier could be added almost as a separate structure altogether.
Following this staged expansion, one could add an identical tier to the other side. This way, a stadium can grow with fan support and the growth of the city/province it serves. 100 year old stadiums? Lambeau Field in Green Bay is very old, and it started off much this way. It's just one tier though.
Stadium capacity progression:
30,000 (sunken bowl with 15,000 on each side)
37,000 (add 7,000 to one end zone to make a horseshoe)
44,000 (add 7,000 to the other end zone to make a bowl)
59,000 (add a 2nd tier of 15,000 seats to one side built at grade)
74,000 (add an identical 2nd tier to the other side also built at grade)
82,000 (add 8,000 to one end zone on 2nd tier to make a horseshoe built at grade)
90,000 (add 8,000 to the other end zone on 2nd tier to make a bowl built at grade)
As you can see, these later stages may never come to be, but at least one has planned ahead and the facility's capacity isn't bound by its design. Who knows, in 2110 Halifax may need a 90,000 seat football stadium!
Roofing solutions are always the tricky part in my opinion. I haven't come up with one I completely like, but I am partial to retractable roofs shaped like inverted half pipes on parallel runners.