The best you can hope for is that you've done enough research to avoid things that obviously couldn't happen and understand the underlying structure of events as well as you can.
Here are some principles I look at when I develop timelines. You probably won't agree with all of them and that's okay. I tossed some of them out to stimulate discussion and further thinking.:
- Your "if-only" will have downsides. If, for example, Kennedy or Lincoln had lived, the results won't all work toward a better reality. If you don't see the downsides, you probably haven't looked hard enough.
-- Republics will become empires.
- Empires will be ruled by hereditary dynasties.in some form.
- Governments will eventually be run by economically powerful cliques that use their control to get richer and keep anyone outside their cliques from challenging them.
- New technology will get used in wars, no matter how far from warlike it initially seems.
- If someone has a technology breakthrough that gives them an advantage over their neighbors, they will use it to take their neighbors' stuff.
- If someone wants to 'civilize' someone else, they usually end up with as much of those other folks' stuff as the people they are 'civilizing' will let them get away with. (See Puritans and the surrounding Indians)
- Empires, or at least their management, eventually get old, fat and complacent, then get replaced.
- Evil empires eventually mellow, at least somewhat. Evil tends to be expensive long-term.
- Empires that start out caring about their subjects eventually stop caring.
- Empires make most of their subjects militarily nearly useless if they last long enough. That eventually reduces the quality of their armies and especially their ability to bounce back from major defeats.
- And there are always barbarians lurking outside the walls of any empire.
- Economic competition usually leads to wars.
- Bipolar struggles between coalitions of nations each led by one dominate nation usually end with one of the competitors winning, then disintegrating itself in a few decades.
- There are periods when events are pretty much going to happen a certain way--World War II after mid-1942 and periods where a few hundred or a thousand men can tilt history--the aftermath of World War I, for example.
- Disease, famine and long-term demographic trends are usually far more potent than wars in determining the large-scale scale structure of history. Wars usually just demonstrate facts on the ground that were determined long before the war.
- Where the economic and technological balance in a war is reasonably even, wars are usually decided before the first battle by the economies of the combatants and what they choose to produce. See World War II, where any lingering hope for an Axis victory was scuttled by the fact that all three major Axis powers spent way too much money and scarce material (steel, rubber, fuel) on extremely expensive battleships that couldn't have had a significant impact on the war.
- A good enough demagogue can stir up trouble between two groups of people even if they have lived at peace for generations, intermarried, etc. And once that trouble reaches a certain point, it can quickly spiral into mass murder, even genocide. (See India/Pakistan during the partition for example)
Feel free to add your own principles or dispute any of these in the comments.