A 4 Use might be a little too much. I participatedd in the 3-use MTC back then, and I can say that a 3-use is still possible to build for everyone and that more is just ...derp, mostly.

I'd rather suggest to use the current tmx mtc topic. "No flat blocks"
Or some other topics like: "Left turns only"

I'm building an offroad monster non-MTC map right now and it has a 4-use and various 3-uses. It's not derp. You just have to know how to do it.
Another example of 4-use is tmj's 3-minute map called "... and you will know us", 4-using the entire start section.

But you have to keep in mind that an MTC should be open for everyone, not just for advanced builders

right, but it shouldnt be challenging only for beginners
it should be a challenge for anyone

haenry says:
But you have to keep in mind that an MTC should be open for everyone, not just for advanced builders

It IS open to everyone, 4-using a section isn't overly complicated.

Yes. This sounds awesome. Do it. I like reuses. Premotes creativity. This is the longest sentence in my post.

What about an MTC called Less than 90 degrees where you build a track which doesn't go vertical by 90 degrees or more.

sonixerik says:
What about an MTC called Less than 90 degrees where you build a track which doesn't go vertical by 90 degrees or more.

In canyon. Really. That's your challenge.

sonixerik says:
What about an MTC called Less than 90 degrees where you build a track which doesn't go vertical by 90 degrees or more.

i.e., no U-ramps and no loops?

the themes should be in "positive" manner: The track MUST contain this and that. Or: The track MUST have this and that property.

Ok then. How about Creative 90 Degrees, where you must get the car at least perpendicular to the ground once without using U ramps. You can still put it wallrides/loops, but the car must be at min at a 90 degree angle to the ground and have gotten there with out the use of U ramps once in the track. This can, for example, be done with barrel rolls (such as hitting a ramp with only two wheels, or with the large statues in stadium (Joost's Brace Yourself).

If this is too complicated the degree min could be lowered...

And perhaps, as an addition/clarification to that, wallrides are not prohibited in that transition, only the use of the quarter pipes to change the car from horizontal to vertical.