Let's see...
Grand Valley: Brilliant layout. Only modification needed would be more runoff in some corners, and the removal of that ridiculous bridge/tunnel combo (even if it is majestic). The uphill corners after the first hairpin (both versions) are dicey in a faster car but I don't think that would be a problem.
Apricot Hill: Only problem I can think of is that the hairpin comes very close to the pits, so paddock space might be an issue.
Midfield: Getting the topography right here would require a lot of work and dynamite. Other than that it's probably fine.
Trial Mountain: This NEEDS more runoff just about everywhere. The giant berm on the last corner could go too.
Autumn Ring: Might need some more runoff in some areas.
Cape Ring: I actually think the loop and jump add something, but more runoff would be needed. The northeastern section (between the loop and the south course intersection) also seems a bit narrow to me.
Deep Forest: Needs more runoff (are you starting to detect a trend here?)
Red Rock Valley: Would be horribly expensive to build. It'd be awesome to drive though.
Super Speedway: No problems here, assuming you're talking about the GT3-style pit-in and not GT2's cramped entrance.
Tahiti Road: Would need a more runoff. Also, the elevation changes make entering and exiting the front straight a dangerous undertaking in faster cars.
High Speed Ring: More runoff on the unbanked corners.
Motor Sports Land: Seems to have been intended as a kart course, so add a pit area and you're good to go.
Complex String: The major sticking point here would be the first corner - it's almost 180 degrees, you're approaching at Warp 9, and due to the layout of the course there's nowhere to put more runoff incase you mess it up. Wouls make an excellent test course though.
Motorland: It's a bit big for karts, but too small for anything but Kei cars. Who would bother?
Beginner Course: Just a bit cramped, and it isn't even a proper oval. (Has runoff on the outside, the pits are on the outside, and there's next to no banking.)
Test Course (both forms): We already have a few giant ovals for top speed testing, why would we need one more?
SSR5: Since this is a fantasy course I have no shame in putting it here. Add more runoff and it'd be a brilliant course.
SSR7: No place for runoff on the inside and you'd need a lot of room to bulid it. Plus, as soon as you did, the FIA would ruin it with chicanes.
SSR11 (both): See SSR5.
Nameless GT2 track that was used for some exams: Everything about this place seems pretty good to me. Also, raise your hand if you think running the 30M radius section into the 80M radius section would be cool. It seems to have two versions as seen in that braking while turning test in the Legacy.
Grindelwald: Is this run on real roads? If so, racing in Switzerland has been banned since the 1955 LeMans disaster and the government has been strangely resistant to changing that. If not, it needs, you guessed it, more runoff!
Eiger Nordwand: Seems to be run on real roads, so see above. Pit space is also very limited, and having your pit area on the edge of a steep drop doesn't seem like a good idea to me.
El Capitan: Racing in a national park? I can almost hear the environmentalists shouting now.
Costa di Amalfi: Seems like a fantasy course to me so I don't feel bad for mentioning it. Wild elevation changes, too narrow, and absolutely no runoff anywhere.