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I think it is possible. For example, a minimum transaction fee could be set and burned by the protocol. Those burned satoshis could be recycled into new block rewards which could make block rewards permanent without busting 21m. It would have other negative consequences but it isn't impossible (though I'm sure would still be a bruising battle for Bitcoin, judging by recent events). PPC has a burned transaction fee, though with inflationary-PoS.
Yes, It something along those lines, with the minimum fee set to a fixed percentage of the transaction amount may work for Bitcoin, although I have not looked at all the possible issues.
That is an interesting idea. Maybe too controversial to succeed based on the blocksize debates.