At the moment, we don't have anything clearly better. But everyone except the most hardcore Bitcoin maximalists believe some form of Proof-of-Stake will work well enough. PoS will bring along a different suite of problems, but requiring a nuclear power plant's worth of energy will not be one of them.
I would like to bring Open Representative Voting (ORV) into the game. On ORV, every account can freely choose a Representative at any time to vote on their behalf, even when the delegating account itself is offline. These Representative accounts are configured on nodes that remain online and vote on the validity of transactions they see on the network. Their voting weight is the sum of balances for accounts delegating to them, and if they have enough voting weight they become a Principal Representative. The votes these Principal Representatives send out will subsequently be rebroadcasted by other nodes.
Open in this context means, that every account can be chosen as a representative by every account.