Interesting. To start off with, a few questions:
1. What happens with P2Pool blocks? P2Pool blocks (ex:
https://blockchain.info/block/000000000000000001a1a36ada43fb03b4eb05f30cbe94a206a4a457fb4d71e5) pay out the coinbase + transaction fees to a bunch of addresses, proportional to each's contribution to mining the block. If a P2Pool block was included in the Tic phase, would all of those miners be signing new blocks in the system?
2. Does the Bitcoin network enforce proper payouts to the miners of the 144-block Tic phase as a hard consensus rule? It appears from your post ("The new block is considered valid if: ... 2. It transmits the newly generated coins and any fees due in a fair manner") that this is the case, but I wanted to make sure that this wasn't just the validity check that other miners do before signing the block.
3. Who initially assembles the block for the network to approve? Whoever assembles the block gets an advantage (get authority over transaction inclusion, can choose to include their own zero-fee transactions). If you create a system where block n in the Tac phase is assembled by the miner who mined block (n mod 144) from the Tic phase, what happens if that miner is offline? If there isn't a particular miner/signer responsible for creating any particular block, how do nodes resolve which block to sign?
4. How is the 10-minute timing enforced? Do peers on the network simply not accept a block
n+1 if the timestamp from the block
n plus 600 seconds isn't the same as the timestamp in block
n+1, and the timestamp of the new block is within a margin of error of their current (local) clock? What would prevent 'whiplash' where miners/signers see it in their best interest to sign the new block as soon as it is proposed (regardless of whether it has actually been 10 real minutes since the last block), which causes them to be creating blocks that, based on their embedded timestamp, appear as if they were made in the future? As they approach the (fairly unknown) maximum allowable clock drift that the large majority of the network will support, they slow down (or even halt) block creation for a while. Even worse, if they overstep the allowable clock drift such that some peers on the network see a new block as barely-in-range compared to their local timestamp, but other peers see the block as barely-out-of-range, is there a desynchronization problem? The cost of performing PoW generally mitigates this problem, since a miner's primary goal is getting the network to accept their block, but in this model (where producing multiple valid blocks has a cost of nearly zero, and a block that gets rejected could have the timestamp adjusted, or could just be re-broadcast later to the network again), these limits could be continually pushed without any significant cost to the miners/signers.
5. What happens when the last block of a Tic phase is contested, with two equally-valid blocks proposed by different miners? Is strong subjectivity lost? If the two miners who created competing blocks for the last block of the Tic phase both had significant mining power in the entire Tic phase, they would actively not vote for blocks that resolve the fork against their favor, which weakens the strength of the Tac blocks against reorganizations.
6. If no block at height n reaches 51% mining approval, what happens? In case of conflict, the block with more valid signatures is considered the valid one.
What if two blocks are proposed, and one gets 40% approval, one gets 45% approval, and there is 15% (current) non-participation. The network decides, somehow, to accept the one with 45% approval. Then, 6 of the previously-non-participatory 15% comes online and signs the block that previously only had 40% approval (making it now 46% to the accepted block's 45%). Does the network then switch to the 46% block? What if another block has already been stacked on the 45% block; do we have any strong subjectivity to convince bootstrapping nodes of the invalidity of the rejected block despite its' higher approval?
7. What about the opportunity cost for miners/signers to create deep reorganizations during the Tac period? The opportunity cost of performing a deep reorganization is drastically lower and much easier to perform during the Tac period, as a majority signature power could collude to create a deep reorganization which they sign together. They could also get unfair leverage on their signature power, because they would
not sign blocks for the entire period they intended to fork later, so that they don't compete against themselves. Miners might not be particularly opposed to signing onto a reorganization, because they don't forfeit their reward by doing so (the reorganization would still pay them the same amount as before).
Scenario: 25% of block signers all agree to, during the first few Tac blocks, send 1000 BTC total to an exchange and sell it. They wait for confirmations on the exchange, sell the BTC, withdraw the value in alt-coins. They then create new transactions which pay out those 1000 BTC differently (back to themselves, and some sent to the block signers who aren't part of the collusion), and create a new fork with these transactions instead. The non-colluding miners are enticed to accept the fork by signing it, because they get paid additional Bitcoin which they don't own under the current valid chain, and they don't lose their block rewards for doing so, because the new chain will still pay out the coinbase + fees proportionally to the miners. The miners also can't simply embed that spend transaction they benefit from in the current chain at a higher height, because it spends a transaction (or transactions) which were already spent in the legitimate chain.
8. What happens if miners/signers sign multiple competing blocks simultaneously? A punitive system which punishes miners who submit multiple competing signatures could be implemented (see the Slasher proposal for PoS), but then the punishment of those accounts would be embedded in some way in a future block, which the miner(s) being punished would not vote for, further antagonizing the mining/signing power.
9. What encourages miners/signers to sign at all, rather than go offline and hope other miners/signers don't? If a miner doesn't have to be online to get their deserved reward from each block, why would they bother validating blocks and signing? If they don't get their deserved reward from each block if they don't sign the block, then it encourages miners/signers to selectively not broadcast other miners' signatures (either so that they can get a higher reward, or to undermine other miners' profitability and get a leg up).
10. What would this change do to blockchains merge-mined with Bitcoin? Since for the Tac period, they can't use regular PoW mining, would the miner responsible for creating a particular block be the one who gets to merge-mine other chains? In the event that there isn't one delegated miner of each block, miners would all propose blocks in which they receive the merge-mining rewards from merge-mined chains, and wouldn't vote on anyone else's.