So, in game terms, what modifiers apply on different types of terrain? If it is raining, muddy, you are sloshing through snow, or you are on a sand dune, how does it effect your footwork? Can it lose you attacks or initiative?
In real world...
on wet ice, you need to keep your center of mass over the center of the line between your feet. It's slick, and if you don't have hobnails or soft rubber, you have no traction. Tennis flats are actually better than cowboy boots, for example, even tho' the cowboy boots are better protection.
On sand, the issue is mostly just being slowed down by mild sinking, and the inability to flèche or lunge quite as far.
On wet grass, if in boots, it's not a major issue.
Dry grass and dry carpet are about equal.
Dry large tile, smooth linoleum, or polished stone: no problem unless you're in hard-soled smooth soled footwear (such as dress shoes or cowboy boots). Then only a problem when you're too far off-line.
Wet tile, wet linoleuym or wet polished stone: we don't fence... it's about the same as wet ice, but with less ability to dig in with hobnails or hard heeled boots/shoes. The slipping happens most if you take normal length steps, so you shorten the distance per advancing, retreating, or sideways step.
The worse the terrain, the more likely a flèche becomes a faceplant and/or an injury to your opponent.
Likewise, the worse the terrain, the more likely a lunge plants you on your face or arse... not as risky as a flèche, but still...
But, in game, we're not able to easily emulate the reach of a trained fencer with a longsword or rapier... 10' with a 4' blade, 15' with a flèche. Except by extending threat, but requiring the fencer to end within 5'...
In game....
The mechanics for difficult terrain are that it costs an extra foot per foot moved.
That phrasing is awkward, but is important because it stacks with crawling.
Very difficult terrain also requires a skill roll at a DC of the DM's choice.
I'd put wet ice at DC15 to DC 20, depending upon what kind of footwear one's in. Wet stone, depending upon algal and mineral deposition, I'd put at DC5 to DC 20.
Slick-but-dry, I'd put at just the movement penalty if in hard soles; non-dry soft soles tend to stick, so I'd not penalize them. For simplicity, call this the default terrain and don't worry until someone throws ball-bearings down.
Sand, just slows you down, most of the time.
Tall grass (more than 1') slows the footwork a bit; call it difficult.
Tall grass with gophers, moles, or prairie dogs, if you slow down, not much an issue, but if you try for full speed... save or reduce move by 5' until a long rest sounds about right. DC probably should be about 5 for light infestations, 10 for heavy.