lol! Well I'm not going to look down on G.W. for a could-be scenario based on a theorized business model deduced on a rumour for an unannounced update on a game

Obviously, it is possible that a new combo like that could be the flavour of the month, either gaming wise or model wise. Who knows? As a veteran player, I'm not worried since I have such a broad range of models that I expect I'll need only minimal models to achieve such a list. As would many veterans do, I reckon. It's no different than the change in power balance introduced by a new army book now, except that it's more limited. For a new player it could hint a good starting base. For a Vet it could offer a new flavour for a few new models.
The options are.. plenty. There's no guarantee it will offer more than we have now, but such a system has the potential to offer more than a single armybook ever has/can. Right now, some armies (including ours) suffer the "best in slot" syndrome. Bleakswords are a rare sight. So are Corsairs. Why? Same for Black Guards and Scourgerunners.
Right off the bat, all units are in immediate competition with one another. It's an extreme challenge to balace them all. In fact, I'd be willing to bet money on it that even you can't pull it off

No offense though.. Neither would I. It would take an immense effort to succeed and eventually it may just prove impossible.
Special rules can help, but it would become a cumbersome, complex book if all kinds of odd combos are presented in the book itself. Special character focused list permits new units to shine in the daylight and special combos.
Most of all, it permits G.W. to focus on a single, solid book and then keep the interest and novelty alive with minor expansions along the way. That's something I would enjoy, at least. Even if I pass on one or two of those lists because they don't suit me game-wise, style-wise or money wise.
I love me a bowl of numbers to crunch for breakfast. If you need anything theoryhammered, I gladly take requests.
Furnace of Arcana, a warhammer blog with delusional grandeur.
"I move unseen. I hide in light and shadow. I move faster than a bird. No plate of armour ever stopped me. I strike recruits and veterans with equal ease. And all shiver at my coldest of whispers."
- The stiff breeze