My current idea:

* There's a scheduled hardcap that goes up over time.

* Miners vote on the blocksize limit within the hardcap, choosing the new votecap. No particular idea for scheduling change. The 2016 block period seems a bit long though, in case of sudden peak load.
(I'd suggest rolling vote over X blocks, enacted Y blocks later (with votes counted from block A to block B = block A+X, the change is enacted at block C = B+Y = A+X+Y). I'm fine with fixed-period schedules too if they span a reasonable time, such as IMHO 2 days - we need rapid peak adjustment. No suggestion on vote result calculation mechanism.)

* Casting votes are free.

* The mean (average) blocksize over the last time period X is calculated for every block, or at the end of every fixed-length period (depending on what scheduling is used for votes).

* Creating blocks larger than the mean but below the votecap raises the difficulty target for the miner (and slightly raises the mean for future blocks).

* The degree of difficulty raise depends on where between the mean and votecap that the size of the given block is (and it follows that lots of votes for large raise reduces per-extra-Kb penalty, allowing for cheaper peak load adjustment if a large miner majority agrees). The degree of increase may be either linear or logarithmic, I've got no suggestion currently on any particular metric.
(Some might think this is an easy way for miners to collude to make large blocks cheaper. If so, you could commit to only pay fee to miners that don't vote for a block size above the size you accept, as a counter-incentive.)

* Question: When the votecap is lowered, should the calculated mean be forced down to follow (forcing a penalty for making blocks close to the votecap straight after the change)? If so, how? Or should it be allowed to fall naturally as new blocks with size below the votecap are created?

This is how miners would pay for actually creating larger blocks, and leaves us with three methods of keeping the size in check (hardcap, votecap and softcap). The softcap mechanism is then our third check to use if deemed necessary (orphaning valid blocks if considered problematically large). This third option do not need coordination with miners, they just need to be aware which block size is accepted by the community.

I can't think of any sensible non-miner mechanism of deciding max block size outside of using a community coordinated softcap, anything else will not work reliably. Too hard to measure objectively and judge fairly.

The community would thus agree on a hardcap schedule in advance, and have the option to threaten orphaning blocks via softfork later on if circumstances would change and the votecap is too large.