Miners not getting paid enough rewards in transaction fees so either the price of bitcoin must double constantly or the hashrate crashes massively as miners quit and makes attacking nodes feasible.
I believe that increment of block size is the best solution because over time, Bitcoin and Bitcoin transactions are getting popular and sometimes we even have moments when the mempool is clogged. My idea is that if we increase the block size, many people will use Bitcoin for making transactions (today many people use altcoins because Bitcoin transaction fees are irregular and usually expensive) and the quantity and massive adoption will result in high transaction fees for miners.
Increasing of 21M supply is ridiculous to my mind. It's a serious threat to Bitcoin network because if they increase the supply for once, this means that there is a green light on that and they'll manipulate the supply for multiple times. I hope that this won't happen to Bitcoin.