How? Well, we can use Club Nintendo, Miiverse, Twitter, and Facebook to our advantage.
Club Nintendo is very easy to do if you already have membership. Sign up or sign in, register your Mario-related games you purchased, and fill out the surveys. They ask questions such as why you bought the game, what are you looking forward to, what did you like about the game, and why did you NOT like about the game.
Why did you buy the game? Tell them you bought the game for whatever reasons you have AND to play as Toad.
What are you looking forward to? Tell them you are looking forward to whatever gameplay-related reasons you have AND you are also looking forward to play as Toad because he's your #1 favorite Mario character (doesn't matter if it's true or not).
What did you like about the game? Tell them you liked the levels, items, whatever reasons you have AND that you absolutely love to play as Toad (if he's actually playable).
What did you NOT like about the game? Tell them whatever reasons you have AND that you're disappointed that you cannot play as Toad (if he's not playable) or you were not happy with how the game portrayed him in a negative light (useless, spineless, etc.)
Miiverse is surprisingly easy to do ever since I started posting there almost regularly. We can go on various communities for the following games:
Super Mario 3D World
New Super Mario Bros. U + New Super Luigi U
Captain Toad: Treasure Tracker
Super Smash Bros. For Wii U + 3DS (Toad for Smash 5)
Mario Kart 8
Mario Maker
I couldn't possibly list them all but you know what I'm talking about. We go to these Mario-related communities, post our messages about how happy you are to play as Toad, how awesome it was to beat the level/race with Toad, how and why Toad is your favorite character, express how disappointed you were that Toad is not playable or that Toad was portrayed negatively (constructive criticism). Fan art also helps so the skilled artists (like YoshiandToad) can draw Toad pictures and unskilled artists (like me) can use stamps to make Toad pictures.
Twitter is easy for people who regularly use Twitter. I haven't touched my Twitter account since January 2013. The only reason I signed up on Twitter was to spam Sakurai with my "Duck Hunt Dog for Smash" with the moveset I made long before I created the support thread here (it was in Japanese too). Anyway, we can use this hashtag #CaptainToad because that's the hashtag shown in Captain Toad E3 2014 trailer. We can use this hashtag along with other hashtags to let Nintendo know that we want to play as Captain Toad in future games like Super Mario Galaxy 3, New Super Mario Bros. 3, and Super Mario 3D Land 2. These games are just examples of main games that didn't have a playable Toad in the previous games.
The goal here is to ensure that Nintendo keeps Toad relevant and important in the main Mario games by including Toad(s) or Captain Toad as playable characters... and Sakurai, in turn, could use Toad's playable roles in those future games to build an unique moveset because those games would be giving Toad more material to work with...
Facebook is easy to use. We can just go to Nintendo's pages and just post that Toad is your favorite Mario character and that you want more Captain Toad games and that you want to play as Toad in future Mario games. We can use the hashtag #CaptainToad on Facebook to show Nintendo that Toad actually have fans.
What do you think? I feel that Club Nintendo, Miiverse, Twitter, and Facebook are our best resources right now... it isn't like Nintendo would listen to fans by visiting message boards. We need to be positive about Toad if he's playable in Mario games and give constructive criticism about the lack of a playable Toad in Mario games.
#CaptainToad