Totally agree that we want fees to be zero for as long as possible (or forever).

What I had in mind is that when network load is not reaching capacity, transactions would be free. If nodes start struggling to keep up, they'd begin charging fees to make things expensive for spammers/attackers. Once load lowered again, fees would drop back to zero. If nodes were upgraded and got extra capacity, the fees for that node would drop to zero again.

This assumes a world in the only purpose of fees is as an anti-DoS measure. If/when fees become the primary economic incentive to mine, well, hopefully at that point mining will be cheap and competitive enough that fees would still be extremely low on a per-tx basis (tiny fees for lots of transactions can add up to a thin margin on top of expenses).

I think there are really two problems here:

1) what should the fees be (continuing to be zero would be best)

and

2) if they do need to be raised at some point, eg to handle a transient DoS attack, how do you communicate that to real users so they know how much of a fee to include?

Also, how do you do that in a fast way such that an attacker can't take out the network quickly. Block based consensus feels intuitively too slow. Imagine how disruptive a 20 minute outage of VISA or MasterCard would be. I actually never heard of such a thing, probably these companies put a lot of effort into reliability and I know they are way overprovisioned to withstand temporary load spikes .... and they don't have the issue of accepting transactions from just anyone.

I'm not sure what the answers are though. Will ponder some more this weekend.