Rammus - The Armordillo
Passive - Bonus movement speed grants him armor, his armor grants him bonus movement speed (no double dipping), but uses the highest recent percentage in the last few seconds (Bonus movement speed causes him initially to be very tanky, late game itemization causes him to move quicker overall, and interesting double-effects happen with Aftershock and Righteous Glory where choosing armor makes him faster, or moving faster makes him tankier)
Q - Rammus enters his shell in a short channel (Think of how people run to start skateboarding - only he is the board), then gains instant movement speed, rather than increasing. Ignoring collision, the next enemy hit is knocked up and dealt (small) damage. Subsequent enemies passed through are slowed instead of knocked up. Recast to cancel, dealing (Armor ratio, distance traveled ratio) damage in an area around you. Much like nidalee spear, this is more about where you started from, not zig zagging for phat stax
W - Passively, rammus deals massive extra damage to targets who have recently attacked him. Activating will sacrifice this passive for it's cooldown, granting CC immunity and Blocking basic attacks at the cost of movement speed. Rammus hides in his shell, not feeling Okay. (2 second duration)
E - Iconic, kept the same
R - Possibly the same as well, structure damage is unique and grants him consistent threat, not burst, in team fights. Could add a "worked ground" taliyah-like mechanic that scales positively with how much you till the same soil, but for the sake of visual clarity not necessary
Unlike Sion, Rammus charge keeps him fast, and low to the ground. It doesn't make sense that he would be interrupted on the first target he comes to. People should have both a reason to block rammus, or get out of the way, but currently in a meta where EVERY CHAMPION has absurd mobility options, and even ADCS and mages are literally insec'ing people and initiating fights, there is no excuse for the character designed to go fast should ever be slowed down. Further cemented in his kit by his ability to avoid CC as it occurs, as it was taken from Ornn but is a mechanic that deserves to be placed on such a character. To be locked down for centuries while 4 ranged can kill you isn't fun, but neither is "Well if i attack this character, i will just die, so thats neat" Notable is the removal of his reflect damage, and also missing is the passive that grants him damage based on his defenses.
Early game, Rammus relies on looking for openings with his kit. His clear speed will be nominal, but he lacks the tools and sustain if a fight is brought to him. His damage effectiveness should be based on his gameplay, not his stats, however as his stats increase, his gameplay becomes more intense and more requiring of coordination - he's not going to zap you like Kha'zix, but he may as well be invisible if you never can react to him coming. Requires some follow up, and requires the enemy he wants to kill to bother paying any attention to him. He can guarantee this once, with taunt, however will struggle to find effectiveness if brought into ranged, extended trades, or against bruisers who can start a fight close, blow for blow without fear, since in order to defend himself he must sacrifice some of his damage.
The one big change is that Q would fizzle with crowd control - counter play for such a strong, no string attached CC has to be offered. If you want to initiate with it, you can get knocked out of it. Or, you can use your normal increased speed to bait crowd control by shelling it out, leaving enemies in retreat - another scenario rammus excels in.
Itemization becomes more interesting - the usual suspects help him with attack damage, but items like Wit's End and Abyssal Mask are a straight synergy to help him stay relevant against AP compositions. While Rammus has some damage options to choose from, tanky doesn't mean invincible. Teams who can effectively stall him out will be rewarded, while a Rammus who gets his team the lead could find himself taking down turrets like bowling pins.