Future expansions.

Very interesting.
Although, won't this create a bias towards one of the creature versions?
I'm not an expert on Heroes 5, but it also has alternative upgrades, and for some reason, I always end up choosing only one – the most extreme one. The other units become unnecessary.

Of course, balance is an extremely important detail. Simply manipulating the stats of the units won't be enough. I haven't played the fifth installment much, so I'm not very familiar with its upgrade system. In my proposed version, all four paths of creature training are useful by default (largely due to the close connection with the initial hero's affinity and reputation, and later – with secondary heroes). It is assumed that, for example, under the command of a mage, the Tears path will directly benefit the priestesses of the tide, snow maidens, and cyclops in battles, but this does not mean that you must necessarily choose only them. Perhaps the player has their own clever tactic, for which the skills of other monsters will be required, for example, ice maidens or deep naga, which he will include in the army of the chosen hero. I would like to see a game with many options, in which, with careful consideration, it would be possible to create a large number of effective combinations by mixing different types of troops under the command of certain heroes. I'm already tired of these identical sets of creatures, and this way, at least some creative process begins.

And also regarding usefulness. I think it can be achieved by endowing the units with truly significant and tactically important abilities, and not with abilities that are rarely used and frankly weak. Based on my proposal, for example, take the priestesses of the tide. If they act as healers and support, then it makes sense to take strong physical units at the elite or master level, which will tank and take the brunt of the enemy attack. They will be able to do this much longer, receiving magical support from the priestesses. In short, the main thing is the essence, the usefulness of the unique abilities of the warriors. If this is the case, then there will be a difficult choice.