Blue Toad
1. How many diverse options does this character have?
So Toad has a long LONG LONG history within the Mario series. This may be the one to show a little bias; heads up.
Firstly Toad's stats; with his superhuman strength and speed, Toad is essentially a rushdown character who'd basically be the opposite of Bowser Jr. in my description; Toad excels at grappling and getting close to the opponent to deal maximum damage. His jumping skills are fairly weak in comparison to the Mario crew, but he'd outstrip the brothers in speed and power easily.
Toad is probably the character MOST associated with items. In game he gives them out when he's a NPC, and when playable he's capable of using them just as well as either Mario brother or Peach. This would make a power up moveset viable for Toad, and is surprisingly something we've seen little of within the Mario franchise with Luigi only covering the fire flower, and Mario the Fire flower and the cape.
Wario's Woods showed his ability to run up walls, opposed to just kicking off them like the brothers, and also associated the mushroom man as somewhat of an explosive master. Lastly this game also showed he could easily carry multiple times his own weight.
Toad's spores are obviously a staple of Peach's B move, but the spores were actually from Mario Tennis. Toad's spore abilities have also shown up in other spinoffs as well such as Mario Sports Mix where they spawn two large mushrooms capable of blocking opponents shots, and capable of launching enemies into the air or creating a shield from them.
Mario Strikers gave Toad emphasis on his leg strength as he can kick the ball so damn hard he sets fire to it.
Finally, Toad is by far one of the most popular Mario Kart drivers, enough to make it into the Top 10 Video Game characters in Japan off just his Mario Kart appearances. He is therefore the best(and notably ONLY veteran) candidate left to represent that series.
2. Is their moves or abilities diverse from the rest of the Smash cast?
Well okay...Toads stats have been semi done at this point unfortunately by Little Mac; a speedy beat down character with super human strength that has poor recovery skills? Yeah that's both Toad and Mac.
Link and Toon Link obviously use bombs, and Peach has stolen his most unique option in terms of both plucking(veggies) and Spore(which is literally Toad himself) leaving Toad with only a few viable unique options.
Arguably his ability to block attacks with mushrooms could draw comparison to Rosalina's Luma technique, as the move is a summonable blocker, Villager could be seen as a new items guy and even more arguably Megaman is a power up user in his moveset...
Wario obviously has his bike in his moveset, which you could also argue would be similar to the Kart.
Thus for truly unique, never been used concepts we have...Wall Walking! No one else has been shown to do this.
3. Has the concept of the character been done before? 3a. If it has been done before, is the character's way of doing it unique on it's own?
As said above, the majority has. However each version has been done differently:
Toad's super strength, speed and low jumping skills may make him seem like Little Mac, but it's fairly clear the two fighters would be very different from one another; Toad is a character that has super strength, yes, but he is also known for item use and grappling(aka plucking things up and holding them aloft his head) which as a fighting style is rather the opposite of Little Mac.
Summonable shields and the use of spores have been done by Rosalina and Peach respectively, but Toad's way would obviously be different; the mushroom would at most be a way of temporarily blocking a move. Spores would work as they do for Peach, a Counter, but could easily do item damage such as temporarily shrinking the opponent for a successful counter in reference to his Mario Kart item; the poison mushroom.
Bombs have been done by the likes of the Links before(as well as the Bob-Omb item) but if we go by the Wario's Woods game for inspiration here the damage dealt would be based on the colour of the bomb used on the victim. For example; a green coloured bomb would hurt standard Link more than it would standard Kirby. Depending on the colour palette of each character the bombs damage would go up.
Items and power ups have been done by others, but Toad's ability to summon forth a poison mushroom, a mega mushroom or a Power Leaf would arguably make him fun to play with in much the same way Diddy was fun to play with his ability to summon banana peels. Power Up suits could be implemented as a way to temporarily buff Toad up. His jump sucks; summon a propeller cap before you need to do a big third jump, suddenly you're sailing through the air!
The Kart is similar in how it'd work to Wario's bike, but visually different. I can't honestly see how to make it different right now, but visually it'd be a lot different.
4. Does the character's abilities and moves provide enough to be entertaining?
Between the power ups and items, the kart, the bomb throwing, the mushroom spawning, the ability to run up walls like Spiderman and the rush down character archetype stats, Toad probably does easily have enough to work with for an entertaining moveset.
He'd certainly be an odd mixture; a small but powerful close range rush down fighter, yet has the ability to block moves with defensive summons and uses items to buff his weak points? That's actually pretty unique.
5. How visually different is the character in terms of body shape and looks?
The only thing that looks like Toad...is another Toad. There is no chance of confusion on the battlefield between Toad and any other fighter on the Roster at current...aside from Peach's Counter, but that IS Toad. Literally one of the most visually unique, yet instantly recognizable characters from Nintendo.
6. Does the moves and abilities fit the character well enough?
If you know your Mario history the answer is a big fat yes. The problem for Toad, you see, isn't a lack of options. It's that he has MANY options, but none are as iconic to him as say...the paintbrush is to Bowser Jr, or Poltergust is to Luigi, thus it's actually kind of hard to pick what direction to take a playable Toad;
That whole bomb thing I described up there could be an entire moveset in itself for example to allow him to be a mid range beast.
The mushroom and spores combination set could make him an excellent defensive fighter rather than an attacker, some may even argue this is a better option due to the fact NPC Toads are often seen cowering, running around in a panic or under attack.
The 'Power Up' set also suits him due to him being the very fellow that not only hands them out, but has been seen using many of them in the recent Mario games(even NPC Toads have been seen using the Tanooki Leaf).
Uniqueness rating: Hard to judge, honestly. There are a few unique playstyle options, although many concepts have been done in the past, I feel there's still enough left to keep this mushroom feeling fresh.
Overall I'm going to give Toad, Blue or otherwise, an A.
Still below Bowser Jr, due to some concepts having been done before and a lot of conflicting choices available for a Toad moveset...hope I've been as unbiased as I can whilst highlighting some Toad options and playstyles people may not have thought of before.