I agree with Istvan comments. The distribution of species (and their genome size) are mandatory to assess a good estimate of the depth of sequencing required to assemble everything. The fact is, to have that, one would have to take into account the hundreds to hundreds of millions of species that are in the sample and follow a kind of Fisher's butterfly approach [*] for each of them, that would lead to build a Poisson mixture model (one / species) with thousands of parameters very difficult to fit. To conclude, same as Istvan: more than difficult (especially a priori).
Nevertheless, rarefaction curves give an information you don't want to discard, BTW, "EMIRGE" can help improving the curve estimation process. As an alternative, what we did in my former lab, was to look at the number of "orphan reads" (= not assembled) in function of assembly effort.
if you find more accurate answers, please share! :-)
[*] Fisher, R., Corbet, A., & Williams, C. (1943). The relation between the number of species and the number of individuals in a random sample of an animal population. The Journal of Animal Ecology, 12(1), 42–58.