Oh boy. Bubbles. Let's talk about her.
1. How many diverse options does this character have?
Not many. Hailing from a pre-Super Mario Bros. NES title, Bubbles is one of the many one-off "retro" characters that would need to have quite a few liberties taken in order to construct a complete moveset. This being said, she falls squarely into the same category as the Ice Climbers, in that she has enough background to play off of in order to make this process go much more smoothly than one might expect.
Bubbles' abilities are few in number, but they do offer some substance. She is able to fire sound waves, and she'd probably be able to make use of Uniras (which could work like the item introduced in Brawl), which would allow her to play the role of a trapper. She also has a
disability, in that she's unable to change direction unless swinging around a pole. Perhaps she could plant down a limited amount of poles at certain points in order to alter how she reacts to the stage. This could be very interesting if she was unable to ever stop moving, for example. Combine this unique movement with a trap-based arsenal, and she can play the role of a unique trapper who sets up the stage as she wishes while maneuvering about it more freely than her opponents can. She can also make use of water and bubbles in her attacks, something only Squirtle really does.
Bubbles could even pull elements from Flingsmash, another Nintendo game with a similar protagonist and gameplay style. Perhaps, for example, she could ignite after traveling fast enough, turning herself into a crashing ball of destruction.
2. Is their moves or abilities diverse from the rest of the Smash cast?
I'd say that yes, they are. If Bubbles' unique method of locomotion were translated into Smash, she'd move in a completely different manner than everyone else currently playable. Sound waves are an attack type hardly explored in the series, with only one of Jigglypuff's moves and an Assist Trophy or two making use of them. She could possibly be unique in dealing damage by simply moving, getting faster the longer she goes without stopping.
3. Has the concept of the character been done before?
Not in any way. A character who is mechanically different from everyone else in the roster due to a unique movement control scheme is something that has yet to be delved into. A very momentum-based playstyle in which the player becomes more powerful but also more uncontrollable and punishable the longer they go without stopping is a very fresh idea not seen before. A literal exaggeration of the high momentum playstyle seen in Fox and Falco would be unique indeed.
4. Does the character's abilities and moves provide enough to be entertaining?
It can be argued that yes, Bubbles could be a worthwhile addition in this regard. Looking at her
Melee trophy, it seems that she could display quite a lot of expressiveness. Swinging around from pole to pole, bumping into walls, and just generally bouncing around like a pinball would make the character quite comical and rather interesting to both play as and watch. Adding in exaggerated, cartoony, visual expressions such as fear (when losing control), pain (when colliding with something), and overconfidence (when moving very quickly without slipping up) would make the character visually appealing and entertaining to behold.
5. How visually different is the character in terms of body shape and looks?
Bubbles is a "bubble fish", which is a completely made-up species. As she originated in an era of simple, pixelized shapes, she's very simplistic in design, which I feel would make her stand out. She's only really comparable to Kirby and Jigglypuff in appearance, but that'd be a bit of a stretch. She'd also likely move and behave unlike any other character.
6. Does it fit the character well enough?
Some of the ideas I've suggested certainly fit the character. I simply suggested translating the gameplay of
Clu Clu Land into a Smash environment, and I think that if pulled off successfully, it would be a natural fit that would also produce a rather unique character.
B+ or A-