Air Assault Infantry: Science fiction in 1940 but mainstream in 1980. Infantrymen carried by a flying vehicle, the helicopter mostly, and delivered directly on or behind the frontline is a revolution in the pace and dimensions of war. Fast, can be armed, and delivers troops on the enemy. This unit could be the ultimate blitzkreig spear lance, but its weakness is the fragility of helicopters, fuel cost and the complexity of flying them.
Mounted Infantry: Infantry is the backbone of armies, but feet can only bring soldiers so far. Thus armies used horses to propel infantry forward faster to the frontlines and carry part of their equipment on them or towed, but troops would dismount near the enemy and then maneuver to combat. This was the actual state of cavalry in the west, more resembling dragoons or "foot cavalry" of the imperial age than charging knights. The same units later would use trucks or apcs and even helicopters to maneuver and support them just like horses. Some of this type of cavalry units still exist today in rugged and logistic poor regions.
Artillery: Sometimes individual artillery units were gathered into great formations to concentrate the firepower on a single spot and coordinate on a greater scale the logistic and security need of the artillery. Such was done during WW1 on French frontlines and certain sieges during WW2 on the Eastern Front.
Flying Boat: Hydroplanes were invented right after proper military aircrafts were capable of take off and landing on solid ground. Ships would carry or tow one, or several, to recon farther than optics could allow to see. But when empires owned a number of islands as such they couldnt have enough patrol ships and hydroplanes, the larger flying boats were invented. Largely the same if not for size, but they used their belly to sea land and float on. While ignored largely in history, these planes were so versatile they speculatively account for half the sunken tonnage of ships during the Pacific War.
Coastal Battleship: Battleships are the best form of power projection in the early 20th century, but are not cheap. And so foreign dreadnaughts or old cruisers were sold to poorer nations to stand as battleships in name, only rarely upgraded from its original conditions. But the worth of prestige has its weight in gold.
Breastwork Monitor: When mentioned many would think of the USS Monitor of the American Civil War and disregard them as obsolete. But this line of ships has seen battle in almost all 20th century conflicts that included water fronts: The island hopping campaigns of the Pacific War was often covered by monitors to complement artillery of destroyers and cruisers; in WW1, the Belgian troops held onto a strip of land under the watch of several monitors; and the riverways of many nations were guarded by them. Simple barges turned into gun platforms or older cruisers armed with battleship guns and heavy armor, a monitor has multiple origins but the function is simple: firepower.
Anti Air Light Cruiser: The rise to power of carriers and naval aviation shifted focus from ol'gunboats to anti air warfare, the powers of Japan, USA, Britain observed this and a specialized cruisers was born. Sometimes it was just a large destroyer with more anti aircraft guns. Not necessarily a fail proof mean of defence but it will be an aid until the rise of precise anti aircraft systems.