I would argue that you can't break the 21,000,000 coin guarantee and still have Bitcoin so the inflationary model of PPC is out. Instead I'd suggest a hard fork to a hybrid POW/POS system with target block times of 2 minutes. POW blocks would still target 10 minute block times and receive the block rewards + transaction fees. POS blocks would target 2.5 minute block times and receive any transaction fees included in their block. This solution continues to reward those who have invested millions in Bitcoin specific hardware while encouraging Bitcoin holders with any significant stake to run nodes.