On the Mario story posted here, there is a lot more information that points to this being a big deal.
From the original story at VGC
So not only was Nintendo going to hold a physical event at E3 (something they haven't done since 2012), they were also going to talk about the movie and theme park. With this and the games coming out, I think there is the real possibility Nintendo is going all in on Mario this year. This could also point to
Fighter Pass 7 being a Mario character.
I should note Nintendo has done this before. For Smash Wii U/3DS, they released a new stage for Mario's 30th Anniversary. Back then, stages were not bundled with characters. The stage released in September (the same month as the anniversary) and was the only new DLC stage not tied to a new character. Now, stages and characters are tied together, so there is a real possibility Nintendo will add a Mario character as FP7 to coincide with the 35th Anniversary.
https://www.youtube.com/watch?v=IOdhFajSdj8
I'd argue the addition of the ARMS character supports this. If ARMS was to promote a sequel, then it would reason Nintendo would also request a new Mario character for Smash to promote the 35th anniversary. If it wasn't for ARMS 2, then it could point to Vol 2 being very Nintendo focused, which would, again, probably mean we'd get a Mario character in conjunction with this event.
My guess is Nintendo will pick a character and add Mario content around it. So a new Mario stage of some sort, music from many different games, and a spirit board likely made up of old Mario artwork (classic sprites, artwork from old games like 64 and Sunshine). As for possible canidates, there are quite a few
- Waluigi - Popular request and a mainstay Mario character that they can use to promote the series as a whole.
- King Boo - We haven't seen any spirits for Luigi's Mansion 3 despite its success
- Toad/Toadette/Capt Toad - Like Waluigi, can represent the series as a whole and don't have the assist trophy issue to contend with. Also Nintendo puts Toadette in a lot of things now
- Pauline - Same as above. Also helps she was in the first "Mario" game and would be interesting. Not sure how she'd fight
- Geno - Obligatory mention. Would be "hype" and add some of the Mario RPG content in Smash
I should also note we haven't had a Mario Maker update in a while and didn't get on in the last direct. Updates are still planned for the game and we haven't heard anything since January.
See here