When someone wants to build e.g. dark oak treehouse with different type of wood or a cobblestone base with mossy cobblestone, it would be great to be able to build with replacement from the same oredict

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Tbh, maybe neither. Making this work for the builder would be a pita and it shouldnt be done widespread as it might make the styles look horrible and bypass a lot of the intended costs.

If the balance is the issue, maybe it should be a config file opt-in option. We can already change builder speed in the config file and other things.

Is this still being worked on? If so, does it still needs placeholder blocks or do you plan to swap most blocks within a oredict like the OP asked?

Two completely different things:
this is about replacing "the same" things = does not change the value/price of the building
the one you mentioned is about being able to build a completely redesigned building without doing it manually = change the value/price of the building

Ability to change within an ---> oredict <--- when building a schematics

Oh, you want to restrict the change so you can only swap wood for wood or stone for stone?

I hadn't been thinking about the building cost as such. I haven't noticed buildings with outrageous costs and what constitutes expensive could vary with the mods included or even basic gameplay. Back in Vanilla, mossy cobblestone was "expensive" since it only came from dungeons. I always saw the costs as being a quantity thing.

This will need to wait for 1.13

Take a look at #1452

Wouldn't this break progression? Many schems use different types of wood as a way to gate progression, since they're rarely found together. If you could replace all of those with one type of wood, it'd be much easier.

I read this as proposing the ability to replace blocks on any schematic, I had a similar but different proposal:
Support for a tag-based placeholder. As an example, it could be tuned to minecraft:planks, minecraft:logs, or similar, then a copy of the 'wooden' style using it would accept any type of plank, including modded ones, without any extra effort.
Pros:

• Automatic support for anything a mod tags as a certain block type
• Allows schematics to fine-tune requirements somewhere in the middle of the existing placeholders
• Can be tuned per-style, so styles where it would break progression just don't use it
  Cons:
• Buildings using it could end up made up of a mishmash of blocks
• May be tricky to implement requests for it (To solve this + above, maybe make a building pick a single block to replace all placeholders of the same type with when created)
• Styles need to be updated if they want to support it

Your placeholder idea is planned, basically, an overhaul of placeholders is planned (thanks for comment/ideas). During placing there are probably gonna be two modes - strict which will allow you to change blocks using builder's defined map (eg. builder doesn't care whether you will use spruce or oak wood but wants not eg. jungle wood, and free mode - which will allow you to swap anything for anything (you can limit yourself to similar/same tags only, a better block picker is definitely gonna be added)