Sorry if this post got anyone's hopes up for another chapter. The next chapter will be out on Wed at Noon eastern standard time.
I want to start by saying that I am not complaining about people posting suggestions about alternative devil fruits for Markus. I appreciate it because it means the people making suggestions really care about this novel. That honestly makes me extremely happy.
I'm posting this to get it out of the way and do a little explaining of my thought process on why I picked the Goro Goro no Mi for Markus. Believe me, I did a lot of research before deciding. I didn't pick the Goro Goro no Mi just for the 'cool' factor. Though that doesn't hurt!
First and foremost. It's the first extremely powerful Devil Fruit that he canonically would have easy access to. Even if he sat on his hands and did nothing, Luffy would defeat Enel and leave Enel vulnerable. It would be a piece of cake to obtain the fruit at that point with the serum. As for the other Logia's he runs into before the Goro Goro no Mi, they're interesting but 3 would be pretty impossible to get.
First, the Moku Moku no Mi... Only a complete idiot would steal a Devil Fruit from a Marine Captain. That's just a recipe for being hunted down like a dog everywhere. Second, the Suna Suna no Mi, Crocodile's fruit is okay but its major weakness is freaking WATER. The One Piece world is more water than land. Picking a fruit that can get bitch slapped by anyone with wet fists is just a plain bad idea. As for the Yami Yami no Mi, There's no way that Markus would be strong enough to kill Blackbeard by the time he got to Jaya without me making him pretty OP early on, something I want to specifically avoid. Lastly, the Mera Mera no Mi, I'm a fan of Ace and really liked him, I would never have my character steal his fruit. Additionally, it would super piss off Luffy and make Markus an enemy of the Strawhats and all their future allies.
Second, while all devil fruits have potential, we all know that some fruits have more potential and/or power than others. Clear examples are the Magu Magu no Mi VS the Mera Mera no Mi or the Ton Ton no Mi VS the Kilo Kilo no Mo.
Third, the potential of the Goro Goro no Mi rivals or even exceeds most other devils fruits that exist. For example, the show states that Whitebeard's Gura Gura no Mi is 'the most powerful' because it could potentially destroy the world. I'd say the Goro Goro no Mi is just as powerful if not more so. Electricity is a fundamental force of nature itself. It could be used to remove the charge between atoms to disintegrate anything and everything. With a snap of his fingers, he could turn the entire planet into atomic particles.
Some facts about plain lightning to keep in mind.
Travels at approx 220 million MPH, approx 1/3 the speed of light, approx 60,000 miles/second
Generates up to 1 billion volts
Generates up to 200,000 amps
Reaches a temperature of up to 50,000°F (5x surface of the sun)
So what can we take away from that? Sure, he's slower than the Pika Pika no Mi but I honestly don't think that Kizaru can actually travel at the speed of light simply because he wouldn't be able to tell where he was going. He literally wouldn't be able to see and would likely be unable to process so much information using Haki. Then there's relativity to take into account and it just gets messier from there.
We can also see that Enel was only using 1/5th of the fruits maximum output with his 200 million volt attacks.
As for the amperage... it takes only 0.1-0.2 amps across the heart to kill a human being. Lightning can have enough amperage in a single bolt to kill 1 million people. Of course, that's normal people and not the monsters in One Piece, but even if they took 100,000x as much amperage to kill a single bolt would still have more than enough.
As for temperature... Akainu's Magu Magu no Mi should only be able to reach a temperature of around 3,000°F. That means lightning can be 16x hotter than magma. And yes, the temperature of the surface of the sun is around 10,000°F. More fun facts. The human body vaporizes at around 2,100°F. Stone vaporizes at around 6,700°F. Get the idea?
As for Luffy being immune to the Goro Goro no Mi, I don't believe that to be entirely true. Yes, Luffy is immune to straight lightning strikes and pure electrical attacks. However, Rubber is NOT immune to heat, it actually reacts pretty badly to it. Luffy would also have 0 chance of reacting to its sheer speed. Luffy only beat Enel because Enel was a cocky idiot. Also, plot armor.
Here are just a few things he could do.
Electromagnetic Railgun - He could easily launch projectiles at astonishing speeds to deal massive damage. Speeds of Mach 7 or approx. 5,400mph
Electrolysis - He can use electricity to split water into Hydrogen and Oxygen then use a spark to ignite the hydrogen for a hydrogen explosion.
Electric Detection - He can detect anything with electricity inside of it. This means literally ANYTHING but especially people. Enel used this in combination with 'Mantra' (Observation Haki) to keep his eyes on an entire freaking island.
Electrokinetic Flight - He can fly, 'nuff said.
Plasmokinesis - He can create and control plasma. Dude can make Lightsabers using his bare hands.
Particle Acceleration - He can charge and accelerate particles, basically he can fire particle cannon beams from anywhere, like his eyes for coolness sake.
Bioelectricity Manipulation - He can control his body's electricity to increase his strength, speed, and stamina to ridiculous proportions. He can also DRAIN the electrical charge from another person's body and kill them. He could even potentially turn anyone into a puppet just like Doflomingo can.
Electrical Telepathy - Yes, he could even use electromagnetism to read people's minds and communicate with them using telepathy.
These few things just begin to scratch the surface of what the Goro Goro no Mi is potentially capable of.
I hope this helps give some insight into why I've chosen the Goro Goro no Mi for Markus and where it can potentially go.