< possible spoiler space >
"Friends"
Admittedly I have not read most of the headers here so this might possibly have been suggested already but I think the writers blew it if in fact the sweater and Ross are not red herrings, doubtful at this point
I believe that they should have set up this season after it had become clear that Rachel was pregnant and not Phoebe that Rachel had gone through an early mid life crisis what with Phoebe already having children albeit not her own and her two best friends getting married, her inability to have a serious long term relationship etc. all this culminating in Rachel becoming artificially inseminated at yep you guessed it from the same sperm bank that Joey donated to though making sure that her anonymous donor made absolutely no references to "Knicks Rule!"
However what becomes clear in the next episodes is that Joey has indeed gone back to the sperm bank and created another donor profile, which just coincidentally is the one that Rachel has chosen.
This becomes clear to Joey who eventually shares what he knows with say perhaps Phoebe. I believe that this arc could have been as interesting and lasted as long as the sub-plot when Monica and Chandler kept their relationship a secret.