Although I like the idea, "weighted by market cap" is a bad measure.
I'm not sure though what would be a good way to select them. Also, small market cap coins tend to grow faster than coins with a large market cap. I can imagine different funds, some with small cap coins, some with large coins.
Of course, this year altcoins showed crazy increases, many went up 10,000% in a few months. If you have a fund that invests in 50 different coins, a few of those would lead to a very nice ROI. Especially at this point an index fund can safe you a lot of work, as it's an impossible task to safely invest in that many different coins on your own, and keep them in wallets under your control.
I would invest in a crypto index fund, if it's created by an established institution, not a random ICO where an anonymous person wants to be trusted with hundreds of millions of dollars.
You could have different funds in the crypto space, just like in equity.
Apart from large cap, mid cap and small cap funds, you can have thematic investments. One fund could invest in coins which have an anonymity theme. I think the first step is for an ETF to be established. Then you can have different fund variations.