I was thinking of a way to reduce the blockchain size (e.g. for bitcoin, but could be used for other altcoins as well), for a higher number of transactions per second. Would it be possible to use some kind of rolling blockchain? By this I mean that only the last x blocks are saved in the blockchain (x sufficient high to be sure there are no forks). To get the value of an address, all transactions for this address until block x are accumulated, and added to a new block, with a special information transaction, which is not signed, but just specifies the public address and the amount. Every client and miner can verify that it is valid before the old blocks are removed, assuming it all starts at block 0. This would mean some overhead for new blocks, because of many addresses with this info transactions, but this would be roughly limited to the number of users on the net, because even if someone uses multiple addresses, all addresses with 0 coins on it, would be deleted.

The concept needs some more details, tests, analysis etc., like it might be a good idea to add the special amount transaction x/2 number of blocks after the last transaction or amount transaction happened, to make it more secure. But I wonder if this concept is already known, or discussed somewhere, or even implemented for some altcoin.

Another idea: if for one year no transaction happened on an address, the amount could be automatically transferred to a pool from which the miners gets paid. This would require that the user sends some update transaction regularly to reset the one year counter, but would prevent that too many lost addresses accumulate over a long time. And wallet software could automate this. Only for cold storage it needs to be done manually once a year.