Despite being some of the Doctor's most iconic foes, Sontarans where pretty underwhelming in the 1.12 version of the Dalek Mod. Right now DM 1.16 is in a pretty early state, but in the future i expect Sontarans to be added, here how:

I thought of two versions: a modern one and a classic one.

Modern Sontarans:

-26hp (Due to their muscular body i'm pretty sure they are more durable than a human)

-15 Armor Points (I know, it seems crazy, right? As much as a full set of iron armor. But why give them high health when we know that their durability is due to their armor. Also, this way it will be easier to implement the weakness i'm gonna talk about later)

-3-4 damage (Depending on the difficulty)

-Really fast attack speed (This, combined with the rather low dph, can give a whole different feel to this foe)

And finally, i decided to give them a special ability: in the episodes Poison Sky/Sontaran Stratagem the ability for Sontarans to deactivate weapons was featured, similarly, in the mod, Sontarans could be able to give a cooldown to all inventory slots on the player that would last just a few seconds. And perhaps their 'rare drop' could be an item with this exact ability.

Classic Sontarans:

About the same as their modern counterparts except they have 12 armor instead of 15, deal higher damage (5-6) but attack way slower and they don't have the ability.

But where do the Sontarans spawn? Mainly in the Overworld at night, just like any other mob, just rarer. But the main place to fin them is the Sontaran Battleship: a structure of remarkable size with the shape of a sphere. The interior would have three levels: the top level, an armory with most of the loot; the central level, some sort of control room; and the low level, the cloning room.

The latter is something i want to focus on: Sontarans are clones, so why not put the spawners in a place where it would be sense to put them? The room is, of course, circular, and has four cloning batches as well as a middle section to stand on. Inside the batches there are the spawners, as well as a new fluid, the very reason why the player would decide to explore this dangerous structure: Cloning Fluid. It's function is very simple: it makes a spawner faster when touching it. Useful for farms but also dangerous if used uncorrectly.

So, this is it; just a few ideas to reintroduce a mob present in the past version of DM.

An image of the Sontaran Battleship:

I just realized i totally forgot to talk about the weakness...
Hitting a Sontaran on the back will deal 1.5x damage and ignore the armor value. Nothing too complicated.