A Pippinless Scenario - If Pippin never left the Shire
If Pippin never left the Shire . . .
Just a few example. Assume all the way through it somehow worked unto that point
He would have never matured
Frodo may have never gotten to Bree
Black breath in Bree
Merry may have not gone with the fellowship either
Someone else would go with the Fellowship, and I'm not even going to go there, because the consequences would be beyond my reckoning. So supposing it was just an unsymbolic 8-member fellowship (or the 9th died, causing depression)
The hobbits would have no one to cheer them up
They may not have turned around on Caradhras, all dying
Movie verse: Pippin would have never distracted the cave troll, so Legolas couldn't kill him, so they may have been delayed further so the Balrog got them
Boromir might turn earlier and attack the Ringbearer - maybe even get the Ring
Frodo may have left the Fellowship earlier, without Sam, and be unable to find his way
Boromir may not have died, so he could never redeem himself
Maybe more of the Fellowship would have died because of the Uruks
Maybe they would move more quickly, and so Frodo would never leave the Fellowship at all, and Pippin and Merry never taken by the Uruks, so no mark of the Ents . . .
Merry would have been killed by the Uruks, or brought to Saruman
(If so, Merry would be tortured, the Quest revealed, and . . . well, you get the rest)
The Ents may not have stormed Isengard
Sauron would have never been distracted by the Palentir or thought the Ring elsewhere
Merry might have picked up the Palentir and told more
If Merry did get the Palentir, he would be sent with Gandalf, so the Witchking would have never been killed
Pippin could never have saved Faramir
Frodo may have gone into despair earlier, or even insane
Frodo might try to get to Mordor alone
If not, he might have died, and Sam would have taken the Ring
If so, Sam would have been taken over by the Ring, and found/ died
Frodo or Sam or both would have never made it past Shelob.
Pippin couldn't have helped Merry
Merry might not be strong enough to continue alone
Frodo may not have recovered so well afterwards
Actually, he would probably have died in Mordor, if not before
Back in the Shire, Pippin might have been put in jail and the scouring of the Shire might not be successful
Of course, genealogy would be all messed up, and personalities very different . . .
These are just a few things Pippin did. It's quite strange (and I know I haven't covered the half of it, so feel free to add even more in any comments you have ) that so small a person was what made it all work.