I decided to be random today, so here I go. Think of all the cats that would never have been born if Firestar had not come to the forest...
1. There is a slight chance that Feathertail and Stormfur wouldn't have been born. Firestar and Graystripe were best friends, who went out together to train their apprentices. If they hadn't done that, Cinderpelt wound't have run to the river, Graystripe wouldn't have fallen in, and Silverstream wouldn't have rescued him, or even met him. But they still could have met at a gathering.
2. Shrewpaw, Spiderleg, Hollykit, Larchkit, Birchfall, Foxleap, and Icecloud would probably not have been born either. If Firestar had not come, Dustpelt and Sandstorm probably would have become mates, and not him and Ferncloud. However, he could have fallen in love with Ferncloud after he became mates with Sandstorm.
3. Squirrelflight and Leafpool would never have been born (duh)
4. Lionblaze, Jayfeather, and Hollyleaf wouldn't have been born either.
5. Breezepelt wouldn't have been born. If Crowfeather hadn't fallen in love with Firestar's daughter, Leafpool, he wouldn't have had to prove his loyalty to WindClan by mating with Nightcloud.
6. If Firestar had not stopped Tigerstar, he would have killed Bluestar, and taken on leadership himself. Therefore, leading tons of cats into battle, killing them before they could have kits.
7. Whitewing, Dovepaw and Ivypaw would not have been born if Firestar had not brought Cloudtail to the forest.
8. If Tigerstar had stayed leader of ThunderClan, he would probably not have allowed Daisy to stay in ThunderClan (allowing Berrynose, Hazeltail, and Mousewhisker however) so Toadstep and Rosepetal would not have been born.
That's all I can think of at the moment, you guys try and think of some :)