This potential timeline is very similar to the circumstances surrounding Banjo & Kazooie's inclusion into Ultimate. In an
interview done by gameindustry.biz with Craig Duncan, the current head of Rare Studios, it was revealed that development for Banjo & Kazooie began right after E3 2018.
"Minecraft had paved the way for that relationship between Nintendo and Microsoft. I met with Nintendo at the E3 before we announced it. And then we connected our teams, because we thought it seemed like a great opportunity.
We had an initial kick off call with Sakurai-san and some of his team. There was myself, Gregg Mayles... who is the father of Banjo, even though he hates it when we call him that. Andrew Wensley, who runs our business team. And a chap called Paul Cunningham, who runs our partnerships. Those people had a conversation, [Nintendo] talked through what their approach would be, and then that process started.
And then a year goes by of sharing animatics and ideas and character concepts. It was a back-and-forth. We ended up with something that was really great. But the execution came from the Smash team."
With regards to Ultimate, E3 is typically when big Smash reveals happen. E3 2018 revealed Daisy and Ridley, two highly requested characters. As mentioned earlier, Banjo & Kazooie were revealed during Nintendo's E3 2019 Nintendo Direct. If E3 2020 hadn't been canceled, there's a good chance that Geno could've been revealed there, which would've fit perfectly as an E3 reveal given his immense popularity with Smash fans as well as his status as a seemingly impossible character, two things he has in common with the likes of Ridley and Banjo.
This is further bolstered by an
interesting quote from former Rare and current Playtonic composer Grant Kirkhope during a VGC interview regarding Banjo & Kazooie's reveal:
“They were surprised by how crazy it went. The E3 reveal seemed to be gigantic. When I was speaking to Nintendo back at their booth at E3, they were all saying it was a bigger reaction than they’re normally used to for Smash. So Nintendo did recognize that it was a big deal. They were surprised. And we all were: it was such a huge release for all those fans who’ve been waiting so long for a new game.”
If Nintendo had taken note of how big of a reveal someone like Banjo was for Smash, and perhaps wished to replicate that kind of reaction at next year's E3, then a highly requested character like Geno would certainly fit the bill, would he not?