The Prussians at Sedan in 1870 (indeed throughout the entire first part of the Franco-Prussian War) largely prefigured the use of artillery during WWI. They might not have had as many guns as were eventually on the Western Front by 1916, but their artillery at Sedan looks and acts a lot like early WWI artillery. On the other side the French Chassepot rilfes also demonstrate a level of firepower significant enough that it changed French shock tactics and, under better commanders, would have probably completely offset the Prussian artillery advantage.
Both side also made significant use of railroads for logistics and troop movement. Prussian infantry tactics had changed to small(er) unit advancing in open order and firing prone. The Franco-Prussian War really looks much more like WWI than it does the ACW, despite being only six years later. The French even make use of early machine guns.
The Balkan Wars of 1912 and 1913 were likewise a "dress rehearsal" for WWI, particularly as it was fought on the Eastern Front. These wars saw some use of automotive transport, machine guns, modern artillery, and even aircraft for scouting.
WWI was undoubtedly the largest and most definite transition from Napoleonic to modern warfare. There are many "firsts" in WWI that never happened in prior wars. Having said that there isn't the sharp discontinuity between the Napoleonic tactics of the ACW and the tactics of WWI that many people suppose. The period from 1965 to 1915 saw a pretty constant evolution of the technology and doctrines employed in European armies that culminated in the massive conflict of WWI.
Soooo, yeah, first of the "transitional wars", that sounds right, feels right.
Now, let's see.... first war with industrial level indirect artillery? Not the ACW of course, even 600 cannons (by the Confeds at Gettysburg iirc) were meant more as direct-fire shelling not as indirect fire with spotters and preplanned arcs or whatever shooting for miles.
Eh.... .....I do seem to think that's WW1, but maybe not? WW1 was where the re-emergence of something like Nappy's artillery cities came as a shock beyond all expectations. Anything earlier obviously didn't make enough of an impact to create expectations from experience.