Hi..i just had a random thought and wonder why such a system has not been mentioned or tried.
so, the block chain is bloated and is likely to always bloat after a given number of blocks under the current structure. So i had a thought while reading through some alts, i saw how they imported balances from other chains here is my suggestion:-
After every 50000 blocks, there are 10 "Maintenance" blocks. During this time the app automatically parses the chain for all balances and prepares to import them into a new block. To avoid gaming issues, NO transactions are processed during maintenance blocks. at block 50000+11 whichever solves it adds all relevant balances to a block and alll previous blocks are "pruned/deleted".
That means for every year of blocks, the chain has 1 hour maintenance time whereby it summarizes the year and starts afresh.
Well, you are mistaken in that it has not been mentioned or tried. In fact, Satoshi covers pruning in the original whitepaper.
Most recently, in Gavin's
https://bitcoinfoundation.org/2014/10/a-scalability-roadmap/ post he touches on the current state of implementation of pruning, and it is likely much further along than you expect.
There are several threads that go into detailed discussion about the methodology of implementing pruning effectively and efficiently, and it is very actively being worked on, just search the forums for 'pruning'.