Rusty Slugger for Super Smash Bros. Ultimate
Now let me start off by addressing the first thing that's going to come to your mind. "This is never going to happen." My response to that is that you are 100% correct in that assertion. It's not ever going to actually happen. But this is just for fun, and I personally would love to see it, so I'm going to explain why I would. First off, we'd have to address that in regards to moveset, there isn't much to work with from the canon for use in a fighting game... or is there?
Rusty never fights in his source material, but the same can be said for characters such as Duck Hunt, Wii Fit Trainer, Captain Falcon, and others. Yet they all still managed to be blessed with an interesting moveset that took some notes from their source material. So I think in the non-existent timeline where Rusty joins the battle, he could have an interesting moveset. For example, he could toss baseballs as a projectile, or toss powerful bombs if the move is charged like an Ultra Machine Man does in a few challenges, at the price of more ending lag. Another interesting move of his could be catching projectiles with a glove, somewhat similar to Villager's pocket. And for his side smash, it could work well as being similar to Ness', being a baseball bat that deflects projectiles upon a hit. I also have a more out-there idea regarding his down-smash. In his source material, Rusty's Real Deal Baseball, he is more or less never seen without his desk, which can also be seen in that image. So perhaps this is a strange idea, but what if for his down-smash he pulled out that desk, and swung it along the ground before it breaks or disappears? Yeah, that's a tad out of left field, but I think it could be interesting. My last idea for a move is for his up-special or recovery move. In some challenges in RRDB, UFOs appear as common enemies, so what if Rusty's recovery move had him grabbing onto a summoned UFO to get back on stage? Of course, this doesn't cover all of his moves, but I think it can give a good idea of what kind of fighter Rusty could be.
Regarding alternate costumes, he could have four colors of Rusty, and four for Inuji Darumeshi, who is the Japanese portrayal of Rusty Slugger.
I'll repeat that it's obvious Rusty is never going to get into Smash Bros., but I do believe he could make an interesting fighter in this what-if scenario. Thanks for reading my little spiel.
