SPROUT MEGAPOST
So, in a Wendy's promotional video, we saw this -
OH, THE PASTABILITIES
Many seem to think this is a Growth-related item from Golden Sun. Here is a Dark Dawn version for clarification -
This could be the case. It has been proposed that these could be use to grow climbable vines, and ladder attacks are a promoted feature.
NEW TEAM HEALER?
It's been suggested that this is, perhaps, a new team healing item. I do not agree with this. What we saw was a 1v1, and in the 3DS & Wii U versions, team items do not spawn EVEN WHEN TEAM MODE IS SELECTED for 1v1. They only spawn with 3+ players.
Additionally, while many items have been redesigned, this would be an extremely drastic redesign that goes from a floating, stationary orb we've seen for 2 entries to a ground-planted sprout with an orb that does not have streaks like Team Heal has.
NEW HEALING ITEM?
This has also been proposed as a possible explanation. One aspect that is NOT being discussed about this item is that it would need to be plucked in order to make sense, and my hypothetical proposition is that it may have a similar effect to the Healing Field item if pulled. This is, however, pretty redundant since this item -
...Can presumably just be thrown down. It should be noted that despite the visual similarities between the sprouts, this item has gone unchanged. It is advertised on the Smash website as-is, has been seen in demo footage, and was shown in a Nintendo Direct.
Additionally, I've seen it suggested that a sprout would have to be associated with healing. However, this may not necessarily be the case, as Villager has a sprout, and a sprout icon exists in the Wii U item quantity slider. It is as much associated with healing in Smash as it is with tree growth and menu options.
OTHER IDEAS
Probably not since Ramblin' Evil Mushrooms exist, but it's worth considering.
I'm not sure how this would function since in-transit the item is seed-based. It would make sense if the item spawned as a deku-nut like item, got thrown, latched on and sapped, but spawning as a sprout doesn't go in line with this.
The Flora Sprout from Super Paper Mario was used for mind control. Not a terrible suggestion, in my opinion, since maybe you could throw it and if it connects your opponent's controls are reversed, but it would be very unusual to see a random, one-off Super Paper Mario reference with only a vague backing concept.
This item has been spotted in Ultimate already, but an underrated possibility is that a Sprout is simply an ALTERNATE of this. Grass is considered a capsule item, much like crates, party balls, and capsules - and some of those have alternates. However, my issue is that grass does NOT have a bubble surrounding it in Smash Ultimate based on the image that surfaced. It would be redundant if it wasn't an alternate in the same vein as present Crates or sci-fi barrels.
VERDICT
It would seem to be redundant if it were simply another healing item when the Healing Field almost supersedes any purpose it might have, especially in tandem with existing healing items like the tomato, heart, & so forth. There are some options from other games, but they range into being obscure, or impractical, unless the Sprout item itself is just visually weird.
I honestly do think that it could be an item you pull that grows vines to climb. Would that automatically make it a Golden Sun item? No, it could hypothetically be Mario-related instead, though that's more associated with stems. If it exists, would it confirm Isaac? Not necessarily, since items like Lip's Flower exist, though between that and the speculation over the Rathalos image, I'd have to concede that Isaac's chances would greatly increase if an item similar to something from his games is in after his AT and most Golden Sun references were outright removed in Smash 4.
Now, if we keep making assumptions, do we assume the leak is true if this somehow confirms Isaac? I think it should be looked at skeptically. If Isaac is in, we'll know based on his mural position, as is the case with ANY character announcement. Isaac being in doesn't confirm the Grinch Leak, but I'd agree it would start to tilt pretty heavily in favor of it, and it would be confirmed/debunked by the time the mural updated since we know where to look for Isaac.
I currently don't know what to think, as is the case with the actual mural/Grinch Leak itself, though the mural details Vaanrose has shown make me lean fake.