Would that it was quite so straightforward as that. The integrity of such an index would be ultimately reliant on the collective integrity of exchange owners.
wikipedia definition: “In economics and finance, an index is a statistical measure of changes in a representative group of individual data points. These data may be derived from any number of sources, including company performance, prices, productivity, and employment.”
Altcoin exchanges are not subject to regulation and the impact of this is that there's no trustworthy canonical list either of exchanges or of trades and so the completeness and accuracy of the data is unreliable to an indeterminate degree.
Any definition of “representative” can only be developed with reference to this unreliable data.
The statistical measures of change would be based on this hoped-to-be-representative group of unreliable data points.
Still interested?
Cheers
Graham