I'll go with JR as well.
It would have been great to keep Santana, but that ground has been covered more than once. We will never know what Thon might have been, but I suspect it is more than a little exaggerated in some estimations. Abreu would have been great, but the team had a lot of very talented OFs besides him in the interim. And I really like Joe Morgan, but while he would have been great had he stayed, he would not have been enough to push the team over the top in his prime years. There were too many holes on the field and flaws in the management to overcome. So at least he went to Cincy and became part of one of the greatest batting orders ever. BTW, I don't have the time, but I wish I could devote more to studying Morgan's home-road splits over his early career, both in the 'Dome and at Riverfront. Because they are some of the most schizoid I've ever seen.
JR was striking out 300 guys a year and would have continued to terrify batters for awhile, plus pitching into the 1980s - still part of The Second Dead Ball Era - he would've piled up some impressive stats. JR would have had a good shot at some strikeout records, and the HOF. And assuming he could've kept his effectiveness to mid-decade, you'd have him, Ryan, Joe Niekro/Bob Knepper, and Mike Scott in your rotation.
Very nice preview.