As with any champion design, the most important thing about them is their "Power Fantasy".
The Power Fantasy defines everything there is to know about a champion from both lore and gameplay perspective. For example Orianna is a dancer by nature, performing a constant tango between her and her ball. Draven is a showoff who loves to stand out and perform for the crowd. Sion is a gargantuan undead warlord who shakes the very ground under his assault. Jinx is a mayhem sowing excitement lover who blows up things for fun before speeding away chasing the next thrill. Etc. And every champion has one.
When a champion really makes you go "Wow..." and takes your breath away, it's pretty much always because they have a power fantasy which matches what you personally like.
When it comes to Rammus however it's difficult to really pinpoint an exact Power Fantasy for him. What does he do which could really be put down as THE reason to play him.
He taunts people... He rolls around really fast... He's exceptionally tanky... He makes enemies kill themselves when they attack him... He stacks Armor... He makes Earthquakes happen... He says OK and grunts a lot...
Personally, I would put down his Power Fantasy, his character defining attribute, as being that roll of his. He can curl into a ball to move at incredible speeds, however becomes less tanky from doing so (incapable of getting the Armor and MR from his Defense Curl). Other things improve and flesh out his playstyle, his taunt is a big part of his gameplay but it doesn't define the character quite like his Powerball does.
And that's the real point behind this mock fan-rework. (And as the "Pt. 1" in the title implies there is another armor stacker rework coming soon after)
There is in particular one play which all Rammus players like to do which involves charging in with Powerball and then flashing directly onto the enemy, ether catching them off guard or using it to bypass minion waves. Which I wanted to make easier by adding a new skill dedicated entirely to that purpose.