I've suggested this before but maybe it's worth mentioning again.

Split bitcoind and bitcoin-Qt into completely independent components. Bitcoind stores a local copy of the blockchain, connects to the p2p network, relays blocks and transactions, and serves blockchain information to clients only. Bitcoin-Qt is a client application only that connects to a trusted bitcoind node. The default installation would install them in a pair, with bitcoind running all the time as a unix daemon (Windows service) and Bitcoin-Qt started on demand. Only one instance of bitcoind is needed on a typical home LAN, all the clients can just connect to it.

Once this is done Bitcoin-Qt can just focus on being the best client it can be, while bitcoind can focus on the blockchain and p2p network. It should also be extended to serve MultiBit, Armory and Electrum. Once the reference implementation is fully refactored into a client/server application it should make any attempt to develop an alternate implementation easier.

I don't have enough programming skill to do this, but I'd donate to anyone who does.