A common suggestion that gets thrown around here is that people want a way to carry their early-game units out into the mid-game and late-game, whether that be through a power spike at rank 6, additional ranks for all units, or some form of prestige levelling system for units. While no one of these ideas is terrible, I believe it misses the forest for the trees, somewhat. For the most part, what people are actually missing from units that they had to replace is the “feel” of that unit and whatever unique role it served on the battlefield.
To this end, what I think would be a better solution in the long term, and not involve having to introduce broad new mechanics or heavily rework old unit balance, is to eventually get to adding additional “upgrade” units that fill and improve upon the niche of a previous early-game unit.
For example, as the Salamander is an upgraded form of the Flame Trooper, so too should there be a heavier version of the Puma, an improved Arsonist, a Hornet with even more RNG to gamble on. What people want isn’t necessarily to keep using their Hunters forever, they want a unit that feels like a Hunter but doesn’t completely fall off the moment you leave the Boar Badlands.
Obviously, this comes at the natural labor cost related to introducing brand new units, but I think this solution more closely aligns with already existing design principles, doesn’t involve the introduction of any new mechanics nor any overhauls of current ones, and is just simply more interesting.