I would like for Caesar's Legion to makes it's way to the Common Wealth. My idea for a good "howdy" would be a full on assault on sanctuary. Give me three in game days of non stop defense, where after every attack I have to go out and rebuild turrets, reset traps, maybe fix a structure or two, and even provide medical aid to my settlers. You can sleep and wait but if you leave at all during the three days you can potentially lose a random companion, hopefully piper. I want to feel the desperation of me slowly losing building material and other supplies like ammo, of course if your a high level this shouldn't be an issue but it still be cool. After you defend your self I'm sure CL will respect your strength and it would be cool to have a random encounter with a messenger, like Vulpes, commending your strength. Maybe bring back the faction reputation scale we had for New Vegas.
Side note, bring back death claws nests/packs. I came to the wasteland to fight the strongest enemies.