Replace GWB with Zeppelins. Zeppelins have extremely long range, but, like Missionaries and such now, can suffer attrition (historically, lost more airships to storms and weather than to the enemy). Very low bombing effect, but, as said, very large viewing radius.
Machineguns get an AA effect against Triplanes and Fighters (low-flying aircraft). Regular AA will come later, but should be placed in the Tech Tree at about the same time as Bombers.
Historically, regular artillery (light German 77mm field guns, specifically) took out more of the WWI 'landships' than anything except mechanical breakdowns. Give artillery a slight bonus against Mobile Units - they were death on cavalry as well as being used even in WWII against armor.
Any mass use of air power requires major ground support and construction. Therefore, the unlimited basing of aircraft is a another complete game breaker, but easily remedied:
City = maximum of 3 aircraft based, plus 1 aircraft for every 5 population points over 10.
Airport = + 3 aircraft
Fortress = maximum 6 aircraft based
In other words, a 30 population capital city with an Airport can base 10 aircraft, but that's not going to happen very often. To sustain a major air offensive, you'll need to expend Great Generals or rely on massed aircraft carriers lurking off the coast.
The real problem is that the sequence in the Industrial/Modern Tech Tree is so FUBARed now that the relationships between weapons and units is skewed. Just for examples:
GWI represents infantry armed with smokeless-powder magazine rifles. Historicaly they started appearing about 1885-1890. At virtually the same time, the modern gas or recoil operated machinegun appeared. The Gatling Gun, always a very unreliable weapon, only lasted for about 20 years at most, unlike its near-universality in the game. Indirect fire artillery techniques did not appear until 1905 (Russo-Japanese War) and were not perfected until 1917 (mobile forward observers, mathematical registration, ultra long range fire with meteorological data calculations). Which is approximately the same date as the introduction of the Landship.
On the sea, the Ironclad dates from 1860-1865, while the modern submarine comes along about 1900 - 1905 and the modern destroyer, which was originally called the Torpedo Boat Destroyer because it was designed to counter the surface version of the torpedo-carrying submarine, starts appearing at about the same time or a few years earlier. The modern Dreadnaught battleship dates from 1906. The aircraft carrier with an air group that can do any real amount of damage doesn't come along until the early 1930s (earlier ships could launch and land aircraft, but the aircraft didn't have any effective weapons until dive bombing and air-launched torpedos were perfected)
In the air, the 'triplane' or pursuit aircraft, was perfected between 1915 and 1918, the multiple engined bomber in 1918. Neither had any appreciable effect against ground troops, cities or ships except in EXTREMELY favorable circumstances - Billy Mitchell demonstrated GWB against anchored, unmanned battleships, and 'triplanes' or pursuit aircraft spent most of their time shooting down reconnaissance aircraft and artillery spotting balloons - and each other. The first semi-effective bombers were Zeppelins, which bombed London during WWI but, while terrorizing some of the population, managed to do about as much damage in two years as one WWII air raid did in one night.
Effective antitank guns start appearing in the early 1920s, and only 15 years later does the tank as represented in the game appear. Antiaircraft guns (75-77mm) were improvised with vertical-firing mounts by 1918, started getting really effective by the early 1930s. The introduction of Radar, by the way, should result in a very much more effective air defense, but has no such effect in the game at all.
In short, correct the Tech Tree shenanigans and a lot of the Unit - Counterunit problems will solve themselves...