^Peach has a lot more options now than from the pre-Melee days. Nintendo really did need to scrape the barrel back then as they even needed to give Toad to Peach for a move (also where did the Peach Bomber come from?).
If Toad was made playable, I see either one of the following happening to Peach:
1. Toadsworth is her new standard special. Same function as Toad; however, it is just Toadsworth. Perhaps he can use his cane to counter attack?
![Awesome :awesome: :awesome:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
He serves better as Peach's assistant as he is more closer to Peach than Toad is.
2. Peach gets a new move from
Super Princess Peach. Vibe powers or even the Parasol (charging shot attack?) can be used here. SPP was not represented in Brawl, and I believe it might have been due to the fact that it was released during Brawl's development.
3. A heart? Spinning hearts have been associated with Peach and can work as a defensive counter in some games (like in MK:DD).
3. Another Toad. Toads are generic...it is a possibility that while Toad becomes playable another Toad will still be her move.
Generally speaking, he's not that important, no matter how often he's shown up in those small roles. At least within the core Mario series. It's probably the combination of non-core titles, Super Mario Bros 2, and NSMBWii that have given him so much attention. Only in SMB2 does he really shine and isn't just a placeholder.
You have to admit that he still sticks out in the core series after the main four. I'll admit that he is not always important; however, he is still an integral part of the Mario series. After the main four, it is generally Toad himself who comes next (Yoshi comes close, but he appears less). Toad is notable; however, he and his roles are generally overlooked in some games by some people. If anything, Toad represents the supporting role of the series and his frequent appearances (some with little importance) is part of his charm.