I like this idea, but I have trouble with the idea that what could be made from such a stock wouldn't be any better than what is available already.
The "idea" of breeding pure lines and creating F1 hybrids is to make something better than what is available. It is in fact how most popular veggies and fruits have been created, and what enables the food industry to support the ever growing population on the planet. It's what IMO all cannabis breeders should* do too (breed IBLs for breeding and growing, create F1 hybrids for growing, or improve IBLs through backcrossing traits into it). Many people have heard of Mendel's theories, but the man who put it into practice in a way it's still done today is Wilhelm Johannsen's, check out his
Pure Line Theory and his discovery of heterosis/hybrid vigor.
What it comes down to is that you have, over those 50 strains combined, a huge gene pool. When you recombine those genes in different combinations the magic 'can' happen and with 50 strains to pick from there are plenty of 'better' combinations.
To create a new variety through crossbreeding, one first has to create variation. By mixing more than two strains, you have more variety in which to do a pheno hunt, but you also end up with more variation to breed out. By stabilizing (inbreeding) two strains "before" you cross them (which is kind of the whole idea of F1 hybrids) you end up with a stable hybrid.
One of your indica plants: AaBbCCDdeeFf
One of your sativa plants: ZzYyXXWWvvUu (using different letters for illustration purposes)
Breed those into pure lines (which takes more than a couple of generations)
The resulting true bred indica plant: AABBCCddeeFF
The resulting true bred sativa plant: zzYYXXWWvvuu
Cross those into an F1 hybrid and you get: AzBYCXdWevFu in ALL the offspring.
F1 is not stable for breeding but should be as stable as it gets, in other words "uniform", for growing. If that's not the case, you're not supposed to take that F1 to F2 (as it will recombine genes in the parental combination rather than the recombination that can lead to hybrid vigor but also to something completely new), but go back and stablize the parent. Ideally, self those to test for homozygosity.
If you have that many strains in a seed bank, you may have great candidates (for F1 hybrid parents or recurrent backcross parent) already. The easiest way to test that is to self them. If you self a homozygous plant you get little to none pheno variety. If you see traits segregate in the typical F2 ratio (1:2:1 and incase of complete dominant 3:1) you know the genes for those traits are not homozygous in the parent.
Also, keep in mind backcrossing saves the genes, not necessarily the genotypes. The only way to do that, is breed them true, as then every generation will have the same homozygous genotypes. Backcrossing could be used to for example breed a taste/smell trait from one in another.
*So we can all swap IBLs and make the world a better place.