I don't think you have to worry about people providing voluntary signatures only. It doesn't make any sense as a rational strategy.
If you are trying to force higher fees on people, then you want to control txn inclusion in as many blocks as possible. To do this, you refuse to sign blocks unless you are the PoS miner. Say there are 5 signatures and the fifth signer mints a PoS block. You are the minter and you have 10% of active stake to work with. To control blocks, you veto anything that doesn't make you the block miner. The base probability you are the PoS minter is 10%. The proportion of blocks you can veto is 1-(9/10)^4=34%. There will still be the other 56% of blocks that get through. For these blocks you won't be able to control anything. Going the veto route raises your effective control of block content from 10% to 10%/(10%+56%) = 15%. This would be scary if veto's were costless. However exercising a veto means giving up rewards. Previously, you earned a reward from 41% of blocks. Now you will only earn a reward from 15%. To justify this, you need to have increased fees by 2.7-fold. However you have only decreased non-controlled block space by 34%. Whether this is worthwhile or not depends on the price-elasticity of demand (defined as % change in quantity / % change in price)
http://en.wikipedia.org/wiki/Price_elasticity_of_demand). For the strategy to be worthwhile, we need a price-elasticity of greater than -34%/270% = -0.125. This is not plausible. Only very cheap essential goods like salt have this characteristic. I'm pretty sure people will give up coin txns before they give up salt.
If you own more than 10%, then the math will get less attractive. However, if you own a lot you also have to think about the asset value which we haven't factored in. Presumably if fees go up, then price goes down.
Intriguing argument, though I don't claim to know enough about microeconomics to fully evaluate it.
The two issues were (1) should we incentivize stakeholders to maintain full nodes by penalizing them with demurrage fees, which involves the complex mechanism of blacklisting and voluntary signatures, or just letting stakeholders collect fees for the work that they do is enough, and (2) whether the protocol should enforce rules that prevent or disincentivize stakeholders from attempting to collect too high fees and thereby exceedingly enrich themselves.
Earlier I was a little concerned about letting stakeholders collect fees, as opposed to letting miners collect fees, because the work that the stakeholders have to do in order to enrich themselves is costless, while the work that the miners do is costly. However, another aspect of this is that active stakeholders have to maintain full nodes, which is useful in preventing network attacks on the cryptocurrency (mentioned for example
here).
You claim that we shouldn't worry about (2), so the simplest option is to allow competition by having the miners re-solve the block, and additionally if a substantial enough amount of stakeholders are honest and calculate the fees via the recommended client rules (like the reference Bitcoin client) then hopefully the price of the fees will be pushed towards that target, because of the competition.
I still wonder about ideas for collective punishment to stakeholders if their fees are excessive, especially ideas that don't involve infinite monetary inflation. Maybe mandatory caps on the fees that are enforced by the protocol are also possible. For now I tend to agree that the simplest option is best.
About (1), if we take the complex route then I think that my suggestion of protocol rule that says that only addresses that control 1 (or 10, or 100, ...) consolidated coin(s) and aren't blacklisted are eligible for the lottery, so using a database would be just one possible implementation. You haven't replied yet to post #145 about storing heartbeats versus storing txns in the blockchain ("txns approach is less bloated" ?), if you think that we'd need to store many heartbeat signatures in order to take advantage of blacklisting, then this becomes even more burdensome. So far I still favor the simple approach of having only mandatory signatures.
It could make sense to allow all the (say 3) lucky stakeholders to include txns in their block, otherwise an attacker who tries to generate empty blocks will have an advantage.
I tend to agree with your post #134 (and with coblee) about having only one kind of block. The PoW difficulty could re-adjust simply by taking the time difference (as Bitcoin does), meaning that the re-adjustment is affected both by total hashpower and by stakeholders' participation level, and we cannot measure each individually. I think that each block should be created by one miner and three stakeholders, the monetary inflation reward should go only to the miner, and the fees should be divvied up between the miner and the 3 stakeholders (1/4 to each). Regarding who may include txns in the block, it should either be all 4 of them, or just the miner and the 3rd stakeholder, or just the 3rd stakeholder. It's a tradeoff between the amount of msgs that have to be sent over the network, and security against the empty blocks attack. Edit: maybe it's better that only the 3rd stakeholder includes the txns, otherwise it would be problematic to make sure that same/conflicting txns aren't added by the others, without sending large msgs over the network.
We should still discuss signing-key/limited-withdrawal delegation, I tried to initiate the discussion in post #112