First off - Poppy is not a tank. Poppy's passive and ultimate actively discourage building defense of any kind. Poppy should be built nearly glass cannon - only one, maybe two defensive items.
Her current kit is one of my favorite to play in the game. Its just fun, and feels immensely satisfying when played well. I am nervous about her eventual VU/GU because I feel like Riot will take her in the wrong direction.
For her VU, I would like her visuals to match her role - while her current outfit suggests she is a tank, she is, for all intents and purposes, an assassin. I think her VU should remove her heavy armor and shield, and instead give her lighter, leather armor (I almost imagine her like a gnome rouge in WoW, though her playstyle is a bit more arms warrior). This armor could have gold plates or gold trimming at places, to tie her in with Demacia (assuming she is still a Demacian ambassador after her update. If she isn't, then gold may be the wrong color). Much of her visual space should be occupied by her hammer. There should be some emphasis on the hammer being bigger than Poppy herself, but the hammer should not be very flashy, given that it was once used as a blacksmith's hammer.
For her kit, I hope they do not change her E and R much. I wouldn't mind some changes to her R to give enemies more opportunity to play around it, but I think the idea of it - tunnel vision, forced 1v1 - are very important parts of Poppy's playstyle that people would miss. Her E could use some bugfixes, but its her iconic ability, so she needs to keep it.
I think the current functionality of both Poppy's passive and her W could be rolled up into one new passive - "Poppy gains X MS, X AR, and X MR when she deals or receives damage. These bonuses increases as she loses health, up to 100% at 75% missing health." Doing this would free up W for a new ability.
Q is important in her kit, but not necessarily a fun or good ability. It could be removed or replaced as long as her new Q or W really brings the damage (Poppy's current Q is one of the most damaging abilities in the game - with full build, it can exceed 1000 damage without the %maxHP damage)
So basically Riot has two free ability slots with Poppy, but between them she has to deal a metric-fuckton of damage. I had an idea for a Q that is a very small (think somewhere between Lay Waste and Dark Sphere in size), very short range skillshot that is targeted similarly to Gnar's Wallop. She slams her hammer down with great force, doing big damage (probably applying on-hits and being able to crit), and knocking aside minions and monsters (and, during her ult, champions that are not her target).
