What if a new mod comes up with a currency and collectable coins are scattered amongst levels? Then collecting enough coins allows certain the player to purchase bonuses such as...
1. Backup power up (kinda like tetris in which you can set aside your current piece and switch it with another.)
2. Radar (a map of gems/powerups is shown on the corner of the screen)
3. Respawning Time Travels (only 5 levels per purchase)
4. Staring with a power up
5. Unlocking new powerups such as the ghost, immunity and boomer.
6. Unlocking locked levels without having to play them.
Take note that coins should only randomly appear in levels, which means that for each level, there's only a number of places that the coins can be found in and where the coins are spawned will be totally random. Also, coins will not be awarded until the player actually finishes the level. This is so nobody can coinspam their way into getting their bonuses.
Also, like most typical Facebook/iPad games, we could also let coins be for sale for around $1.99 per coin.
This has been attempted several times IIRC, and one of the problems is not the system itself, but the foundation it needs to rest on. Since you're suggesting it be a part of a mod, you'll need an already-decent base of levels and fun gameplay to make this kind of thing really work.
But ignoring that, many of those bonuses aren't very practical. As I describe next, it might not be a good idea to go with the kind of bonuses you've listed.
Starting with a new powerup would mean having to build levels based on the notion that the user could have something like a gyrocopter or super jump right at the start. You'd have to design all the levels to work well with this feature, and that's not easy to do without heavily restricting level design.
Again, new powerups are tricky, since the question comes as this: when would you use them? In every level there are the powerups already placed in the level that the player can get without any previous qualifications. If you place 'locked' powerups into the level that the user can't get at until later on in the game when they come back to the level, then that's a big game-changer. If you can't get the best time because you're missing a feature that has to be bought with these coins, then it seems like the coins and these features are overshadowing the skill it should take to beat a level with the best time. In order to combat this, you'd have to, like I said earlier, change up the way the whole game is played in terms of goals for the player.
Respawning time travels aren't as bad, but like I said before, it does change up the game quite a bit.
Radar would have a fairly-limited use in single-player, considering that if you've played a level a few times already you pretty much know where the gems and powerups are. In fact, several of these bonuses seem more applicable to multiplayer gameplay.
Unlocking a level without having to beat the previous one isn't really that big of a deal, certainly something I wouldn't spend a coin on. It's not completely useless, depending on how the mod is set up.
Backup powerup would work if it basically means a second powerup slot where you hold another powerup. Only problem is the one that seems to keep popping up: too much of a game-changer.
Okay, so I understand that may have been a bit critical, but I felt the need to point out that this kind of system would not work easily with these different gameplay bonuses. It'd work much better as a system of aesthetic bonuses, such as getting a new marble skin, skybox, music or ui. Gameplay advances like this do not work well in this kind of system for a game like Marble Blast.