A while ago I quite quickly accepted the possibility of a contraction of Homeric Greek genitive ending -οιο to -ου, as seen in the most dialects.
Now I'd like to argue this once again! First of all, this is not the commonly expected contraction of any dialect; And I'd rather avoid explaining strange forms by irregular sound contractions. In this post I opted that -οιο seems to reflect a form PIE *-osio where -ου reflects PIE *eso or *oso. The first one would correspond with Sanskrit -asya, while the second form agrees with forms like the one we find in Gothic -is.
Of course our high sense of lumping everything together as some kind of Proto-Greek would make us want to delete once of these forms. But I think we might have to consider that maybe Proto-Greek at its very earliest stage already had dialectal variations, which may have even been carried over from Indo-European.
It might be worth noting that -οιο is not only seen in Homeric Greek, but also in Mycenean. Bonfante went as far as suggesting that Homeric text was originally a Mycenaean text in his article "Homer Text is Mycenaean". I would not go this far, throughout Homer there's such an enormous amount of evidence that the main dialect was Ionic, that it seems absurd to claim that it's Mycenaean just because of a Mycenaean-like genitive and evidence of a once existent waw (which all dialects must have had at some point, not just Mycenaean).
I'd sooner think that -οιο was loaned from Mycenaean because it is metrically a lot more pleasant to work with, for a dactylic hexameter. But that idea is sort of denied due to the almost exclusive use of -οιο rather than a metrical whim deciding whether to use -οιο or -ου.
So when we indeed believe that -οιο and -ου are from different etymological sources, which is in my opinion more probable than a sporadic contraction, especially because we find both *-osio and *-eso reflexes in other languages, then we can conclude that there is some kind of switch between *-si- and *-s-, this would be a bizarre assumption if it was only found in the genitive, but lo and behold, such a switch is found in the formation of the future as well!
In Greek, to create the future, you take the verbal root, and add a -s-e/o suffix to it. For example πίμπλημι 'to fill' is a reduplicated present of the stem πλη/πλα- Then the future is: πλήσω. The sigma, in some environments disappears, but this is a post-proto-greek development, and not very important for this discussion.
Now, let's have a look at the Sanskrit future! Sanskrit doesn't insert a -s-a- suffix, like you'd expect looking at the Greek form, but a -sy-a- suffix, and just like Greek it's added to the full grade of the verbal root. For example: tiṣṭhati तिष्ठति 'to stand' stem: sthā- स्था The future is sthāsyati स्थास्यति.
aha another s/si switch. What exactly does this mean? Why does it happen? Why does Greek have both a *s and *si variant for the genitive?
I think this is an indication that Greek and Indo-Iranian languages may have been a lot closer than we think. But why this *s *si alternation seems to be taking place is beyond me.
It might also be worth mentioning that Greek and Indo-Iranian are in fact the only two branches that have a future with *s. Most Indo-European languages don't have a future at all. Why do we reconstruct this future as something from PIE? Because one day someone came up with the arbitrary rule: If it's in a European PIE languages, and an Asian PIE language, than it must be PIE. This idea is nonsensical, it might be helpful to establish true PIE roots, but for grammatical information like this, such rules should not be applied.
Once again, I find it difficult to go as far as saying that Graeco-Iranian was once an actual separate branch, but there's so many unique features to this group of languages not found anywhere else that it's quite idiotic to take the rules found in these languages and propose them as Indo-European, rather than the Graeco-Iranian dialect. Proof of the things we find in Graeco-Iranian is technically absent in all the other languages. Just because both branches have been extremely conservative doesn't mean that non of their shared features are archaic, rather than shared innovations.
 Bonfante, Giuliano 'Homer Text is Mycenaean' JIES 1996
[EDIT] Thanks to Glen for pointing out my terrible misspelling of Giuliano's name. Hah. I should look up whether this article also has the typo, because I was fairly sure I directly copied it.