I've had some experience with my now dead mod... so here are some tips.
1. The levels in the pictures don't look very original, I hope they aren't in the final release(if you would ever finish this). Try to focus on levels, they are in fact are great chunk of your mod. Hire GOOD level builders, not just people who beg to be in a mod. And no, I can't help because Torque Constructor doesn't work for me anymore, and I honestly don't wish to get in this situation once more.
2. Yet again on originality, you don't really need beginner levels such as Learning to Roll, the only levels I would actually be interested in playing are good, original levels, and tutorials are great if you're introducing a new feature or power-up. Keep in mind that the people downloading your mod are people who have most likely already played Marble Blast. You can still do them, but just add your own touch if you choose to.
3. You may have no idea, but this looks A LOT like a clone of every other mod that has failed so far. I still remember designing a few nice levels for MB+, a failed attempt at a mod which went nowhere due to lack of good level design(I made a few levels for MB+, you can find them somewhere on YouTube by searching MB+), and overall loss of interest.
4. Textures are big things, bigger than you may think. I know I wouldn't want to play a mod with unappealing textures that probably took 20 minutes in PhotoShop. Take serious thought into the game, don't rush it and keep developing. Some of the mods I've played have textures that are seriously terrible, I might as well play the game in wireframe mode.
5. Overall, I'm seeing the same things in these levels. A lot of them don't really look like they took a while to make and weren't very decent creations. Time Race right now looks like a mess of platforms, and the tuts looked slightly rushed and lacking of originality.
6. Final tip: Take your time. I hate testing out rushed mods, they're never that good and the levels bore me. Make your mod worth someone's time.
Making mods are tough things. Take a look at the only completed mod so far, Marble Blast Platinum(Don't know about Marble Blast Platinum Quest, and I honestly feel I shouldn't check).
No, I'm not criticizing your mod. I'm not hating on you. I'm giving you tips, and at most a little constructive criticism. Maybe I'll join in the tunes: Great looking mod, can I help?!!
For now, I'll wait and see.
FruBlox here, keep in mind I still check back here from time to time.