I would like a MTC along the line of "Highlight a custom Object" which you naturally have to make yourself.

Not sure how many of these have been suggested before, but:
*Singleton: No block/item can be repeated in the track design. Hard mode: mirrored/functionally identical pieces can't be used to bypass that limit.
*Space-filling: Every single tile within the boundaries of the track must be filled in.
*Dense: At least one tile should have 3 layers of track over it, or a good portion of the track must have two or more layers of track over it.
*Compact: The track must stay within a relatively small [6x6? 8x8?] square of ground.

I like the first one the best--that could be fun. Variations on the "Compact" theme have been done before, and it's always a good one too IMO.

It would be interesting to do a "Compact" theme, but make it all three dimensions (e.g. 6x6x6) and then force a relatively high minimum time limit (1:00? more?) to make sure that a fair amount of track is jammed into a small space...

Here is an idea which maybe could be fun and I believe has not already been done: you can only use a titlepack from a selected list outside of the main titlepacks (standard TM2 packs, esl_comp, the envimix packs). The exact list of titlepacks allowed can be adjusted by the organizers (only reduce to a few to avoid too many headaches?)

An alternative version could be where you must use a titlepack/environment for which you have never mapped before (which would open the use of the standard TM2 titles, for example, for those who have not mapped on all 4 envis already), so that you have to go outside of your comfort zone.

Of course, for even more chaos (fun, sorry, I meant fun XD), this could be used as a 90MC as well

I didn't read all 25 pages so this has likely already been done or suggested but...
"Forward and Reverse"... A set of two tracks with about a one minute run time where the second is an exact copy of the first with only the CP's reversed and the Start and Finish swapped.
If submitting two seperate maps is not allowed then the track could be copied and pasted so the reverse starts at the finish of the first.

We actually had that theme about a year ago. Here's the thread if you wanna check out the tracks ^^
https://tm.mania-exchange.com/threads/4922?postid=45910

No checkpoint, non cutable should be a fun challenge if you pair it with a minimum lenght.

I would like to possibly see a Touge-like map - IE downhill with tight, hairpin curves - as a potential MTC theme if it hasn't been done already.

thought I'd toss around some ideas since we're low on themes

-route crosses the same block three+ times
-track has no obvious route (puzzle?)
-make a map highlighting an unintentional mechanic (bugslide, nosebug, neo, etc)
-difficult map that is <10s
-map is a square
-only platform blocks
-only road blocks
-slow map (<300kph)

these could probably be expanded on/improved

I think we saw an MTC a long time ago which had the three-plus block usage, offan. An the square and platform/road only ones possibly wouldn't be that interesting to see.