I have a random thought regarding unit diversity and specialization especially with the lack in early units' strength (like warrior and rider). I suggest that a unit upgrade system to be implemented. I'm also going to take this opportunity to suggest new units into the game.
First, let me explain this so called "unit upgrade system" I am thinking of. This system will allow weaker units (i guess the ones that cost less than 5 stars) to be upgraded or, in easier words, evolve. However, this is not the same as the veteran status that already exists in the game. What I am thinking of is a class change upgrade or the like that will make a certain unit have different stats or get unique abilities. In addition to that, a unit will have two options to change into.
This is somehow ambiguous, so take the warrior as an example. In this system, the warrior will be able to transcend into either two of different classes by meeting a certain condition (we'll get into that later). These two classes, let's say, will be spearmen and macemen.
To expand on these classes, let us talk about the spearman I am suggesting. The spearman will have the same stats as a warrior. However, it will have a unique ability called "piercing" which will allow he unit to ignore one defense point when attacking. This will allow it to have an advantage against riders and knights as this will prevent them from making counters against spearmen. The maceman, on the other hand, will have an additional attack point.
As for how the units get upgraded, it can be implemented into the veteran status. It can also be in a form of a technology that will unlock the ability to do so. This technology can either be "training" which will branch out from shields or be added into the shields tech, but that will require it to have a new name like "barracks."
This system will prove to be worthwhile as it adds more value into the early units but still leaving the strong units their worth. It will also add diversity into not just the units but also to playing styles and possibilities.
I apologize for the lengthy suggestion, but to sum it up, I am suggesting that the weaker units shall have the ability to be upgraded into two different classes that will have stat bonuses or unique abilities. The method of upgrade may either stem out from the existing veteran status or be in the form of a technology. This system will certainly add value to weaker units and make the game more replayable with even more possibilities and strategies to explore.