'It's popular to play down America's role in WW1 (the majority of schoolchildren don't even realise the vital part they played)but without them the results of both World Wars could have been very different. '
Actually, while Lusitania's sinking was one of the reasons we entered WWI, it was not THE reason. We waited until 1917 to join the allies, two years AFTER Lusitania was sunk. Obviously, it was not enough to get us to help the Triple Alliance, the reason we joined was NOT because of the Lusitania's sinking, but because of economics, and strong ties to the British Empire and France. So, had Lusitania not sunk, we can still assume that in 1917 we joined on the side of the Allies.
'Hmmm.....have we given a thought to the fact that both Lusitania and Mauretania could have been in German hands at the end of the War? Had Lusitania not been sunk (one of the main reasons for America entering the War)and Britain and the Allies defeated, both ships might have been taken as "spoils of war".'
Yes, well, doesn't that sound like an apparent reversal of the horrible treaty of Versailles which let Adolf Hitler dupe the German people into buying his brand of facism?
But back to the ships, Lusitania would have been with her relatives Aquitania and Mauretania raking in profits for Cunard, in a good natured rivalry with the Mauretania. She probably would have been refitted after the war, and converted to Oil Burning. Imperator would have been given to White Star Perhaps instead of the Columbus, ensuring a better transatlantic service for White Star with three 45,000+ tonners, at an average speed of 21-23 knots.
Lusitania would have been kept on the transatlantic run similar to Imperator, while her sister would have been put into cruise service, similar to 'our' timeline. Lusitania would have been on 'booze cruises' in the early 1930's, before being laid up in 1933. Mauretania would be scrapped in 1936, with Lusitania back in service in early 1935, replacing Mauretania on the cruise service. After the Cunard-White Star merger, she would be laid up once again, with her fate undecided. With the outbreak of WWII, she'd be converted into a Troopship along with Aquitania. She'd serve well until it was decided after the war that she was too old for any further service, so Lusitania would be in one of two possible scenarios:
1.(most likely)Scrapped
2.(somewhat likely)Sold to investors in New York as a floating Hotel
Thats just my personal opinion of things.