Here are some ideas:
-Build a map with Maximum speed e.g. 300km/h for Canyon

-Use every Type of Road to drive on e.g. Valley: Platform, small Road, wide road, offroad, tunnel, split road
(might be a bit boring in stadium, due to the low amount of different road types.(platform, offroad, normal road) Maybe exclude it from rules or even better adapt the rules. Maybe like this: count as additional road types: offroad on grass, a section on the blue ground, banked and normal road as two different types of road)
I hope you understood what I mean

this:

ad'gado says:
as many different surfaces as possible. It's good because there's almost no boundaries since you can pretty much build whatever you want. It's also nothing hard to build

ad'gado says:
+This also seems like a good theme for the use of mixmapping and/or envmix

using "surfaces" instead of road types allows more. Also a solution for the lack of road types in Stadium

in addition with haenry's idea of a speed limit. There could be something like "stay above 300km/h" too.

Here's one. It sounds like the the MTC where you build a track starting at the lowest block level and build a track that goes to the highest block level and finish at the lowest block level or vice versa. In this case, the start and finish starts at the same level (ex. the start and finish y coordinate is the same like start x-3 y-22 z-21 and the finish x-31 y-22 z-4) and the track gets to it's lowest and highest point you can build at for at least 3 blocks and the track can reach it's highest or lowest point in your track first is up to you. Another idea is a track where the start and finish is in the same x and z location but the heights are different. Ex the start is located at x-15 y-30 z-20 and the finish is located at x-15 y-3 z-20

There will be an MTC in August. Prepare

Here's an Idea, called a backtracking MTC. Build a track where you travel along at least 5 blocks in both directions and then 5 different blocks from the first 5 in both directions.

drive blocks/objects the way tehy weren't meant to be driven.

imo this will get more creativity. since atm most object users are using them as they were meant for (like yoshi's are most used for tech maps the transition blocks are used to give trantions between different surfaces)

but you know once you start flipping them rotating them with the "+" & "-" buttons you can create totally different transitions and jumps with them. this should force the builder to think outside the box. wich will result in a larger amount of different maps (since If I see maps with object on em like on canyon: most of these maps feel 100% the same wich is really boring sicne the lack of creativity in my oppinion.)

for the rules I'm not 100% sure how to do it, but it'll be like use an object (the way it wasn't meant to be used) every 6s-10s

How about a titlepack MTC? Build a track that requires a certain title pack features objects specifically to that titlepack and not a title pack because of the tracks in solo mode.

So basically just use the objects given in a certain titlepack?
That would be very similar to MTC august, which was about objects too, but embeddable ones

heanry didn't you listen to tobbe on ppo?
DECO SMASHING!

Here's a theme (suggested by s8ndm8n) which most likely wouldn't make enough entries over at TMUFX.

Mini Campaign
- build 3 or more maps with a total length of ~1:45
- max 6 second intros

Judging can be based on the average of all tracks by each contestant.