Here's the important thing to remember: this is yours. If you want it to be part the race, make it part of the race. If I use it in my game, maybe I'll remove that and have my players take an appropriate background to represent it. Everyone will run their campaign the way they want to, so do what works for you.Well, for the added Stealth bonus, it really depends on just how much "oomph" the individual subraces offer. Like you said, a Background can give them Stealth bonuses, and most calibans will come from an Urchin or Street Thief or Family Shame Locked Up In The Cellar type background anyway. So, yeah, I'm okay with it not being a racial trait if they don't really need it.
Okay, let me give a rundown of what SCAG does with the Tiefling so we can see if it helps with the ability scores and the rest. It has a sidebar that introduces 5 variant abilities:Using the SCAG Tiefling update is tempting... but I must confess that I don't really know how to do so. See, I pre-ordered my copy of SCAG before November release, but the shop I ordered it from got only about 3 copies and then sold them all before I got there. And the online store that they got it from has YET to restock the damn thing. So I don't really know the actual way that the SCAG writes up the Tiefling, though I do recall that it gives an alternate ability score modifier, and three optional racial traits - switching out the level 3 SLA for Burning Hands, replacing the "devilish" SLAs with mental ones, or trading SLAs for Flight?
Appearance - This is just narrative and gives you other options for how your Tiefling can look.
Feral - This replaces the normal ability score adjustments with +1 INT, +2 DEX.
Devil's Tongue - This replaces the normal Infernal Legacy with vicious mockery, charm person (2nd level), and enthrall.
Hellfire - This replaces hellish rebuke from the normal Infernal Legacy with burning hands (2nd level).
Wings - This replaces the normal Infernal Legacy trait with a fly speed of 30 from bat wings.
What I'm suggesting we do, then is use the 5E Half-Orc as Caliban by default. I say this primarily because when I design something for a system, I prefer to stick as close to the system as possible. Since Ravenloft took the Half-Orc and called it the Caliban before, I see no real reason to stop now. If we really don't want the default to be identical to the Half-Orc, we can have our own version. It doesn't really matter. The default version, whatever it is, should be an easily-accessible race for the players. Once we have that, we have a few ways to proceed.
METHOD 1: Modular Deformities
We list a number of alternate traits (we can call these deformities if we want to harken back to the QtR article). Each Deformity replaces one or more default traits of the Caliban. Some of them are mutually exclusive. For instance, we have the Ghostly Keening variant trait that replaces Savage Attacks. We also have Cold Blooded that replaces Relentless Endurance. For this example, we have one more trait: Thick Hide that replaces Relentless Endurance. Because both Cold Blooded and Thick Hide replace Relentless Endurance, we can't make a Caliban with both of those traits, but we could combine Ghostly Keening and Thick Hide if we wanted. The benefit here is the modularity. You can combine a whole bunch of different things. Each set of ability score modifiers would be its own Deformity that replaces the default ability score adjustments, as well. Banshee bloodline gets +2 CHA, +1 WIS while the Brute Bloodline gets +2 STR and +1 CON or something like that.
This is, however, far more in the style of 3.X than 5E. The Tiefling variant really only has a few swappable options. Additionally, it poses the problem of how to add abilities without swapping. The Banshee that we currently have lists two abilities, but the Bestial has more. Large amounts of modularity might result in more optimization than the system really wants.
If we did choose this way, we could list each of the variants as "packages" that include a specific set of Deformities. The Banshee package contains Ghostly Keening and Cold Blooded. The Bestial contains Feral Armaments, Thick Hide, and Loping Fiend. And so on.
Honestly, the more I write about this, the less I like the idea. It's just not 5E.
METHOD 2: Individual Variants
This one is kind of obvious. We treat each one completely separately and do with them what we want. Not much to explain here. The real downside is that, again, this isn't how 5E likes to handle things, just from the opposite end of the spectrum from Method 1. If we did it this way, they might as well be separate races entirely.
METHOD 3: Subraces Again
So I think maybe I just went in circles and settled back at the start. Subraces are good. So how do we handle the ability score adjustments? Bah, fine, we just encapsulate the ability score adjustments in the subraces individually. I don't think there's a better way to do it. My suggestion would be to make the Brute subrace functionally identical to the Half-Orc, so that option isn't really being completely removed from the players. And, let's be honest, the Brute is described in terms that would translate perfectly to the Half-Orc's mechanics.