Let me rephrase what @vbuterin is proposing.

Relayer reserves a slot of several blocks.

Relayer takes the merkle roots from the previous update and updates them using the transaction he accepted, and then posts the corresponding ZKSNARKS to the chain, as well as the updated Merkle roots.

Another relayer reserves a slot and processes more transactions (slot reservation may be an auction).
There are several problems:
An obvious problem that needs to be addressed is chain reorgs.
If I post after some other guy, I rely on the Merkle root that was just posted by him. But someone with enough computational power can reorg the chain making the Merkle root I based my computations on irrelevant.
B. The second problem is time it takes to generate proofs. If I base my zksnarks proofs on some Merkle root and if it takes 20 minute for me to generate the proofs, the Merkle roots may be updated by someone else, making my calculations absolete.
So, if it takes me 20 min to generate the proofs, the time slot needs to be at least 20 min, correct? I can not let someone else (another relayer) update merkle roots why I am generating the proofs.
C. Another question which is not addressed is that if there are multiple relayers for the same merkle root, they have to somehow pass the Merkle tree to each other. If I am a relayer, where do I get the updated Merkle tree from? It looks like I need to get it (or at least the transactions) from the relayer that updated the tree before me. How does this happen?
D. Data availability is another problem. As a user how do I get access to the tree? What if relayers withold it from me?