I think part of the problem with call-ins is that they are often jus better versions of what you can get, there are actually rather few truly niche units that have utility through commanders. Let's face it, a Tiger is just a super Panther/Panzer4, it has identical abilities, just superior stats. Anytime you are not playing as a Tiger commander, you would just do the same with those vanilla tanks, just with more caution.
But there are other commanders that offer some competition not by other units bu by abilities: Jaegar Infantry for example introduced no new units at all, but all the abilities were reasonably effective, and applied to most units. Although it focuses on infantry, it allows one part of the fight to be dominated while alleviating pressure on your anti-tank work.
One way of introducing competing commander abilities beside call-ins would be introducing new unit abilities that give unique critical damage. Button vehicle is not only Soviet-specific, but also relatively rare since it depends entirely on A) commander chosen can deploy Guards, B) you upgrade them to DP and C) use the button ability. Surely a commander ability that offers this in other factions or through other avenues can serve as part of a new commander? A Ostheer or OKW commander that offers something like Panzerbuchse AT RIfle to be able to button and take on light armour would be both an interesting angle towards Axis AT tactics and also refreshing: button is not possible other than using upgraded Guards in this game. In other words, introducing vanilla abilities of one faction into another faction that has never had them would be an interesting way to provide new ways to play the game, all without resorting to calling in ever-more powerful exotic badass units.
Even types of critical damage could be used as part of commander abilities (there are apparently a lot of unused ones left from the betas). An ability that allows your units to, say, "attack vehicle crew" could instill temporary or permanent criticals that inhibit vehicle operation in ways other than engine damage: for example this ability could give one of four possible outcomes: driver is injured, momentarily immobilizing the vehicle but is otherwise functioning (can still shoot); loader injured, where the rate of fire is reduced but the vehicle can still move normally, allowing escape; or commander is injured, reducing LOS and accuracy. These are great aids when used in conjunction with regular abilities, all which require regular use of vanilla units rather than replacing them with call-ins that, really, are just there to do more of the same with less difficulty. Moreover, such abilities introduce new tactics, not less.
Another prospective example is a USF commander whose abilities include medium tanks to fire WP rounds to disable enemy infantry. This is pretty much identical to the Ostheer's stunshot, but frankly, since when is that a horrible idea for the USF player, especially if regular weapons like Jacksons and bazookas have horrible performance? Being able to stun an enemy vehicle is not what USF players can do with regular units, not even with existing call-ins, and would be of great benefit and bring conditional balance to the faction. An ability that allows Ostheer to salvage would also be a suitable alternative to choosing a commander that allows for an expensive Tiger; salvaging wrecks is a viable option throughout the match, whereas a unit such as a Tiger depends entirely on available popcap, available resources and having reached CP.
Another part of the problem with call-ins is that, at least for Soviets, most of the units' details such as veterancy and abilities are identical, they are just better versions of what you can bring. IS2 and T34/85 are really just super versions of the T34 in function: they all have the same vet1 ability, capturing points. A commander that offers new and different abilities for vanilla units is already reason enough to try them, besides does anyone really call-in T-34/85s or IS2s with the mind of capturing points with them? A T-70 can do that just fine if not better.
In short, new commanders should start focusing on improving existing units, not more exotic call-ins. Commanders like Mechanized Assault isn't great because it lets you use your units in new and different ways, it's great because you end up not needing to use most of your regular units at all.
Sure they're the developer and they have the final say, but I'd hope that any developer takes their community's opinion into serious consideration when making major changes to their game. Relic has done this in the past, and I believe they're still willing to listen to us.
I'm not in the betas or whatever programmes, but I'd like to cite War Spoils in response and in refutation of your aforementioned opinion (you know where I'm goign with THAT one!).
As for tiers, Tier 3 and 4 are most susceptible to problems associated with call-ins, given that most call-in vehicles are of the super tank sort of variety. Most infantry call-ins are circumstantial (long range mortar, AT and shock infantry for Soviets, MG34 for OKW, paratroopers) and thus less prone to problems associated with call-ins (not to ignore units like Dshk or Panzerfusiliers), not to mention most T1-2 vehicles are either non-replaceable (nothing substitutes an Ostheer halftrack through commanders, except perhaps Urban Assault's Forward HQ) or are replaceable but often at a time where you might not afford just waiting for the CP and resources to get it; Tier1-2 therefore remain relevant to the game. Specifically, Soviet Lend-Lease allows M5 call-in WITH SMG GUards at like 5CP, but that's a time where you might have already built T3 and allowed for tanks, plus the quadmount upgrade requires T3 built as well.