Being able to select what vehicle to deploy can be a strategic choice in the game, providing more variety, playing as France for example, you can choose to use the Schneider CA1 which has more firepower than the FT17 but has less health, or the St. Chamond which is slower than both but has a more powerful gun. Something like the Char 2C can also added to the game as something very slow but has strong firepower and is very heavily armored but obviously is very vulnerable to artillery strikes due to how large and slow it is. These can be balanced by the stronger vehicles having much longer cooldowns and also requiring war bonds to unlock for selection. Germany can get the K-Wagen and LK 1, Britain can get the Whippet and Hornet. Austria-Hungary can get the Motorgeschütz. These are a few examples.
Cavalry should be added as these were still used in WW1, they can be something you can turn infantry into that cost something like 20 war bonds to balance their speed but are bigger targets making them easier to hit.