What I would like to see is a strong preference for large groups over individual super units.
There are endless different ways to do this. Vastly more approaches than there are ways to design a super unit, which basically is a single entity that has some insane amount of HP and firepower.
Higher tech units could be more sophisticated functions for roles you can already do with other units. They might be more specialized, or narrow in their functionality compared to lower tech units. They could be either more or less efficient, depending on the design.
For example, maybe you have a lower tech bomber that flies over the target area and drops bombs. Well, maybe a high tech version of that unit is more expensive and limited, but does that role in a different, more effective way. Such as using guided missiles or laser guided bombs. I am thinking the difference between the F-111 Aardvark workhorse strike bomber and the sophisticated F-18 Hornet. Another way to do it would be delivering a highly potent and effective capability, but to a lesser extent than a lower tech solution. Like having the low tech unit be a giant carpet bomber (e.g. B-52), and the high-tech unit be much smaller, but delivering some extra goodies, perhaps like the F-117 Nighthawk stealth bomber.
Maybe a high tech unit has a role that is just not available on a low tech unit. Maybe low tech units force you to pick between a combat unit and a recon unit. But maybe you can get a high tech unit that is both in the same package. Expensive and sophisticated multirole units will be more fragile by their nature, as they are fewer in number, but they would make up for it in versatility and adaptability.
Maybe a high tech unit is extremely potent at a very narrow role. Such as having low-tech anti-aircraft guns with extremely high rates of fire, shredding air units that get close. And having a high tech AA unit that is a SAM site with enormous range and high stopping power, capable of taking down many types of planes in just one shot. This type of high tech unit is extremely potent, but has a very narrow application. For example, although extremely useful against planes, these high tech SAM launchers might be overwhelmed with large quantities of flying units, and would be totally helpless on the ground.
As another example, maybe your basic tank is a sturdy, reliable workhorse, while your high-tech tanks are flashy and snazzy, but ultimately not as efficient. Maybe the are lighter, with higher-tech weaponry, or missile launchers, or defensive countermeasures, or stealth, or speed, but at the end of the day they don't give as good a ratio of cost to armor. As a result, the high-tech units are more valuable to reuse, and are poorly suited to slugging it out in a pitched battle meat grinder, taking heavy damage and serious casualties.
My ultimate point is that higher tech units need to give players strategic options, rather than being a strict upgrade that just replaces the lower tech wholesale. This is especially true for "super units" because they are as expensive as an entire army.
Super units are a useful strategic option to have in some narrow circumstances. But they should generally be pretty terrible, except when correctly used in that narrow set of circumstances that actually call for a super unit, and where no other tool can do that job.
Honestly it would probably just be easier to not bother to put them in. Super units are an above-average amount of developer effort to create and almost never have a positive effect on the game. Even when done right, as in TA, they don't really affect the game at all.