Thanks for the write-up @fool and for kickstarting the debate around SYN utility. TLDR this discussion will remain very hypothetical in nature until the team provide some feedback on the various trade-offs of certain proposals. My quick & dirty thoughts below.
ETH vs SYN for gas: I'd recommend using ETH as the primary gas token. Using SYN for gas would impact the relative attractiveness of the protocol and slow down adoption as there is currently no spot demand for SYN (other than for speculative purposes). You could introduce an auto-conversion feature like the one Chainlink is implementing, allowing users to pay in ETH/stables which then auto-convert to SYN. That could however lead to a reflexive farm & dump dynamic so IMO the value accrual derived from certainty on tx costs through the use of a high liquidity/low volatility gas token likely far outweighs the potential net demand for SYN from using it as the primary gas token.
SYN as a security bond: similar to the previous point, I think preference should ideally be given to a prime collateral asset which has high liquidity, low volatility and some form of monetary premium/spot demand. That being said, the design of SIN's security model in which a single honest actor is enough to safeguard the system may somewhat reduce the reliance on economic security and could therefore provide some flexibility on this point. Perhaps a dual-track approach could make sense here: allow Agents/Notaries/Guards to post a bond in ETH, SYN or a combination thereof (the SYN portion being overcollateralized and ETH-denominated). This would mitigate any concerns on the part of node operators that don't want exposure to SYN liquidity/volatility.
DAO revenue share: redirecting a portion of fees to the DAO treasury, effectively mirroring the current "admin fee", probably makes the most sense to me from a value accrual/utility standpoint as it achieves a number of goals: i) it creates a sustainable source of funding for all aspects of the protocol's development, ii) it diversifies the treasury into a quality exogenous asset (ETH), iii) it anchors the value of SYN to Synchain's growth and iv) strengthens the notion of SYN’s intrinsic value with a right to ETH denominated cash flows. As the protocol matures and builds up a strong treasury over time, it can start to return excess funds to token holders. This value can flow back to token holders in variety of ways optimizing for capital efficiency, tax and regulatory matters (buyback & burn, buyback & make to improve on-chain liquidity, stake to underwrite additional protocol risk (e.g. insurance fund) & earn yield). There are a variety of trade-offs that will need to be discussed at length here but that discussion feels pre-mature right now given the nascency of SIN/Synchain.
Another aspect the community needs to consider in relation to point 2 is what is the minimum APR required to attract A/N/G into the network? Fee generation at launch will presumably not suffice to allow node operators to clear the minimum return and the delta might have to be plugged with SYN emissions until node operators can live off organic revenue alone. This could have major implications on SYN supply and tokenomics but without the team’s input/guidance on the minimum/optimal number of A/N/G required to run the network it will be very difficult to have any kind of informed debate on this topic. I would argue this last point applies to the whole discussion around SYN utility where I’m sure a number of technical/design choices by the team will drive a preference for one or the other solution.