I wasn't born during the releases of the Original Trilogy so I have always had the PT era filled in for the most part - however none-the-less it is not impossible to imagine the PT if you had only seen the OT.
Much of this is taking into account both the impressions I got from the Original Drafts of A New Hope as well as the original Ralph McQuarrie artwork. Because in 1999 George could essentially create any environment or scene he envisaged I would have imagined the Prequels would have encapsulated Ralph's artwork and the vibe of the initial drafts.
1. The overall setting could have been more ancient than the OT when removed from the Republic Capital of Coruscant and the other 'civilised' worlds. The many 'Tatooine-like' settings would have drawn inspiration of East-Asian culture with ancient temples, monks and a wild yet barren landscape. In many respects the world which Obi-Wan at least begins his journey (see below) would have been like the mountainous regions of Tibet. This would create a cultured yet mythic atmosphere for the Prequels:
http://wwwdelivery.superstock.com/WI/223/1848/PreviewComp/SuperStock_1848R-351227.jpg http://www.sevensummittreks.com/admin/images/113111.jpg http://wwwdelivery.superstock.com/WI/223/1848/PreviewComp/SuperStock_1848R-338322.jpg
2. The Jedi would not necessarily have been centralised on Coruscant with a Temple, etc. They would be spread throughout the galaxy much like Yoda and Obi-Wan were. In this way there were not necessarily in exile but had rather just retreated to even more backwater worlds. In this way, in some respects, Vader having 'hunted down' the Jedi Knights is given new meaning. Furthermore the Jedi would have been like wizards or mystics living on worlds keeping peace and order. Obi-Wan as such would have been much like Gandalf in the Tibettan-like environment shown above, probably with a few other masters and apprentises.
3. In earlier drafts of the A New Hope an old general trained a group of disciples (many Luke's in this respect) to be Jedi. If this concept had been carried forward many young men and women would have sought out Jedi like Luke had Yoda, seeking to be trained as Jedi.
4. Anakin would have been introduced as a spice-freighter pilot as Owen had told Luke. He would inevitably have been found by Obi-Wan and Anakin would have been trained as one of Obi-Wan's students. When the Clone Wars erupted Obi-Wan, as well as Anakin, would have gone on a 'Fellowship of the Ring' type adventure with a clear, and seemingly Holy, objective - this would have more clearly aligned with the 'idealistic crusade'. Eventually they would have reached Alderaan and would have been employed by Bail Organa as Generals to defend the Republic.
5. Palpatine would have ascended through the ranks of the Senate employing political tactics (such as the corruption of 'trader barons' as mentioned in one of the Drafts of A New Hope) and would have ascended to the office of President of the Republic. Inevitably he would have seized power and corrupted Anakin Skywalker to aid in this quest to destroy the Jedi Knights.
6. The Clone Wars would have centralised around cloning technology of some kind, however the clones would not have been the Republics military. It would either be a war to prevent some evil force from obtaining the technology or it would have been a war between the Republic and cloned soldiers.
7. Padme Amidala would have been, as she is in the established films, one of the Republic leaders would sided with Bail Organa and Mon Mothma opposed to Palpatine's growing power. She would have met Skywalker on Alderaan and they would have developed a relationship throughout the trilogy, marry, and eventually the twins (Luke and Leia) are born.
8. Luke and Leia would have been born immediately after the birth of the Empire and Obi-Wan and Yoda would be battling Vader as the Jedi are beginning to be eliminated. Luke would be immediately taken to Tatooine to escape the crisis, while Leia would live on Alderaan with Padme for a while before she dies. The reason for this separation was opposed by Padme, however Obi-Wan convinces her of its necessity.
Many of my ideas were influenced by this 1994 article:
http://starwarz.com/starkiller/2010/05/looking-back-to-the-future-of-star-wars/ (as linked above)
I enjoy and appreciate the Prequels as an important part of the Saga, however the PT created through the OT was never-the-less very different.