A peaceful union of states is a rare phenomenon, as is a division of a state into new neighbouring states that was not somehow preceded by war. After 1945, the average age of states in the world first declined, many new-born ones added to the initial group of relatively old ones. Two quite different explanations are possible. This means, ceteris paribus, that there will be fewer wars within the present UN members as time goes by. Yet we do not know how many states the future will add to counter the present trend towards increasing age of the average UN member.
Traditional decolonisation is almost over and can produce very few new micro-states.
The new states we may get will largely appear by secession or the total collapse and dissolution of states. By historical experience, secession is mostly a bloody affair - and state collapse even worse. The premodern war is then what we saw much of in the middle ages in Europe and today primarily in parts of Africa and Asia. Small armed units are roving around, sometimes fighting each other, but mainly pillaging the land they traverse at the expense of the civilian population, which is brought to starvation at best and slaughtered at worst. Post-modern warfare, finally, was made possible by war going up in the air in the 20th century, and Yugoslavia was the victim of it in 1999.