> fund composed of, say, 50% BTC, 20% ETH, 15% LTC and 15% ZEC

> ... buy the top 10 coins weighted according to some measure, and rebalance once a week

As a relatively sophisticated investor myself, I'm not against the idea, but whether or not I'd invest in it would depend heavily on the implementation. And as of now with that example portfolio, I definitely wouldn't invest in it. Having a large btc bias when all the major cryptos are already highly correlated with btc seems redundant, and rebalancing once a week seems like overkill. Also, marketcap is a poor indicator of returns, in case you're considering using it for weighting.