You are on the right track, thinking about economic incentives and reducing susceptibility to attack. But I think it can be done differently:

• The newcoin blockchain can be secured using the Bitcoin blockchain for some period, simply by periodically publishing transactions on Bitcoin that "checkpoint" the newcoin chain. As long as the Bitcoin blockchain is significantly more powerful, this method should be used. It would cost some bitcoin fees, but it'd increase the security of newcoin tremendously and instantly.

• Incentive must be provided for people to abandon their bitcoins in favor of newcoins. Otherwise, you'll have people with both coins, and you get Gresham's Law impacting the success. Without this incentive, the average Bitcoin holder will take their "free money newcoin" and dump it (sell it, in favor of more bitcoins). With newcoin, you'd want to encourage the opposite effect.

The way to do this would be to reward newcoin holders and users disproportionately to the market value of newcoins. And you'd need honest oracles as part of the system (which is tough to automate).

If the market value of newcoins moves up by x bitcoins, holders of newcoins should be rewarded 2x (or some other factor). The way to do that is through newcoin issuance, or by honoring "interest" on the newcoin blockchain: If you got 30 newcoins in the June issuance, and now it's August, and the market value of 1 newcoins has gone up 20%, you now can spend 40% more newcoins... Those 30 newcoins have grown to 42 newcoins.

It'd take some screwed up programming, and it'd not be capped at 21M. And it'll take a lot more thought than this rambling post, but I think appropriate incentives can be put in place to pull this off. I'm not advocating for it. I'm just saying that it's probably possible.

And all sorts of other changes could be implemented at the same time - like rewarding "sibling blocks" (near-miss blocks which almost solve the difficulty, which still provide proof of work... these are rewarded in pooled mining), reducing the 10 minute block time, alternating between ASIC and PC-based mining, increasing the block size, etc.