Many things are difficult, but that shouldn't be difficult at all: miners just include transactions that pay the most (if space is limited) or that pay minimum X bits, X being defined by the miner, (if the space is not limited). THAT'S IT!
You are assuming a functional fees market exists. This is not yet the case. Most miners are simply selling hashing power into pools. Basically, only a handful of pool owners can define X, and they don't want to constantly play with variables. They want to set up an optimized mining operation and let it run.
In that case, defining "X" is not needed at all. You just always take the best paying transactions, regardless of how much they pay. ("X" is for artificially restricting supply to extract more fees, and completely unnecessary.)
block size limit is simply too small for Bitcoin to maintain reasonable growth, and it will hit a bottleneck just trying to process normal business.
If it's about 260,000 transactions per day, it is indeed too small. If it can be increased, I favor increasing. And also setting in stone the increasing schedule so that no further vote or community decision is needed ever concerning this matter.