The Basics.
Ability Scores (and modifiers)
How Armor Class is determined (and its effects)
Hit Points, Damage, and Conditions
Movement and Exploration Rules
Monster Design
Saving Throws
Bounded Accuracy math
Some of those are very hard to dial, but others not so.
Ability scores should probably not change as such, but skills are already a dial of ability scores: if you don't use skills you have simply the 6 ability scores to differentiate characters' abilities, or you can use skills to increase the complexity. I know that most people tend to see "abilities as innate, skills as training" but it doesn't have to be.
Saving throws can be dialled... it's not that hard to group them (like in 3e) or split them into subcategories based on the effect (e.g. splitting Constitution saves into saves vs Poison, save vs Fatigue, save vs Spells etc.).
Eventually both skills and saves tend to work only upwards, meaning that character options usually grant bonuses to them, while a more complete dialing should include also penalties to keep averages more or less stables.
Conditions can definitely be "dialled". 3e has a long list of conditions, but it would have been easy to take the most similar ones and group them together, ending up with a much smaller bunch of conditions. Splitting them up even further (but they are already quite a lot) would be more difficult because you would need to make a decision for each source of a condition, which one of the splitted subconditions it now causes.
Monster design (by the DM) may not be dialable as such, but if monsters were treated somewhat similarly to characters, and characters were dialable, then monsters would be dialable, for example you could skip feats and skills and focus on simple abilities to design low-complexity monsters, or use all possible rules for high-complexity monsters.
Movement and exploration rules... only minimally dialable but something can be done. First of all movement can be dialled down to minimum by just ignoring speed differences and lump all "difficult terrains" into the same one effect, and dialled up doing the opposite. For exploration you can increase the tracking of illumination, require multiple rolls for perception and trap-finding plus handling traps with detailed description (requiring specific ideas to bypass/disable) and so on, or OTOH you can otherwise ignore illumination (just assume you have what you need), roll perception for the whole party, and deal with traps with a single roll.
You can't have an advanced module that 1/2s the armor bonus from AC and converts it to DR (for example) without it messing up bounded accuracy and HP rules for example. Some rules are best left as baseline for the modules to add onto.
Well armor as DR can be seen as a dial step towards higher complexity, but I would not strictly put it into a discussion of complexity but rather in a discussion on variant rules (which incidentally also increase complexity, but that's not usually the reason why people want a rule of armor as DR, the usual reason being the way they conceptually want to interpret armors to work).
It's the kind of variant rules that can have unforseen effects (and problems) on other parts of the game, for instance the old known problems with rendering some low-damage weapons useless.
But it's been presented before (3ed UA) and there are many groups using such a variant. Just because a variant has consequences doesn't mean it's a bad thing... it only means that a good book will warn the DM on the consequences and suggests some possible solutions. But anyway this is more like a mechanical variant rather than a pure dial.
