Potions, magic beams, redstone,and enchanted weapons, these are some of the things that must to be "terrafirmated".
My idea is to add these "magic things" like realistic things.
For example potions, formally, potions really exist, but these aren't magical substances (well, the magical prefix is relative by the level of knowledge), potions of the past was actually drugs, obtained by the destillation of specific plants and organic substances.
Green fire? Fire that burns underwater? Invisible fire? Exists!, These are (al)chemical mixs.
Primitive explosives... steam machines, optical ilusionism... "these can look magical for the poor primitive Steve that lifes inside of the game."
Redstone (mercury?), in the other side, should be replaced totally by copper, primitive batteries and vacuum tubes... okay, this can seem too advanced, but aren't really so advanced, these can be created with the technology of a medieval town (?).
Potions Drugs (concept):
This has been viewed sometimes in the past, but it stills being a need if terrafirmacraft can be fully realistic (well, with a basic realism).
The idea is simple: to add machines, destilleries... machines for viticulture and alcohol production. Alcohol is a very common drug, easy to produce, its effects are depressant, sedative, disociative, somnifer and hallucinogen, but little amounts can act like a kind of "minuscle temporalstrenght potion" for the player, since it increases the reaction (although badly)and decreases the dolor sensation.
Next there is others drugs as coffee, coca, opium, cannabis, tea, tobaco, each one with negative and positive effects (mostly negative).
This branch could be extended to anabolizant steroids (obtained from the concentration of hormons of animals) that would have a very long and progressive strenght/speed/haste/jump increasing and decreasing;and strong nootropics (from special plants) that could to improve the accuracy, speed, night vision, etc.
Alternatively, poisons theme can be extended and variations can be created (neurotoxic, cardiotoxic, and citotoxic or wither) each one with different speeds of affection and different lingerings (for example, citotoxic is slow but lingering and very dangerous at big place, while neurotoxical is almost instant but its effect can disappear fastly); from plants and animals.
Redstone Electronics/physical mechanisms(concept):
Copper andzinc or silver; copper and iron... and some of acid juice. Batteries can be obtained with low technology easily, simply connect different metals to a potato, and you will obtain energy for do work a clock.
Steam machines also are easy (technologically speaking) to obtain, and generates lots of mechanical energy that can be transformed into electrical using a iron fragment surrounded by copper spiral (an electrical transformator).
So, you can create toothel wheel systems and electrical systems...
Do you want light bulbs? Use carbon or graphite bars inside of a vacuum bulb. How to obtain vacuum bulbs? Technologically speaking, these also are easy to make.
And speaking about bulbs, you can to create primitive electronic displays using vacuum bulbs... you can to build transistors (better said, tristors), diodes, decators... Analogic computing, that is very similar to redstone computing.
Giant machines able to move walls or impulse doors... Primitive tanks... imagine the utilities.
Mechanical mechanisms, not related directly to electricity and steam machines, also could be used for build catapults,etc
Alchemy: from the fire that burns underwater, passing tothe powder that explodes, until the nitrical acid.
You can to mix things forobtain fire that burns underwater and powder that explodes, but mostly important, you can to obtain nitrical, sulfuric, chloridhidric acid, when you has electricity.
Can you to obtain a "magic sun torch"for burn zombies with a blinding flash?, create some of magnesium via alchemy. Invisible fire? Wood alcohol. Etc.
Magic beams:
Build mirrors, parabolic mirrors and the previous light bulbs, and you will able to create since deffensive mechanisms able to burn monsters, until "magical spectres with you shape" that are actually holographic illusionism.
Yes these are only concepts and should be extended, and will be extended progressively. But now is the timeof discuss about its viability, and next, if it is viable, maybe be developed more in detail.