We wouldn't even need to have Garchomp for Cynthia...she can be Lucario's trainer. Though it'd be to see anyone other than Red be the trainer for either Pikachu, Greninja, or the current elemental starter trio as he's usually the trainer associated with all of those. I suppose Blue could take over as the starter trainer, and Serena/Calem could be the trainer for Greninja while Red/Leaf moves to Pikachu.
If there actually is going to be further dlc content past FP2, then this is a pretty likely scenario. All evidence points to all of Ultimate DLC having started from as early as Smash 4, so they would likely need more time to negotiate deals with 3rd parties, design movesets from scratch, etc.
They could use the original design of Red for Pikachu, after all, he uses the original version of Red's Hat as a costume.
Not really, certain characters weren't really known when Smash 4 DLC was being decided upon, such as Joker, and Nintendo wouldn't have likely known how incredibly popular Persona 5 would be until 2017, when the game got released to critical acclaim, so I doubt he would have been on Nintendo's radar that early on.
We also know that Fighters Pass 2 wasn't decided on until partway through the development of Fighters Pass 1, which is why certain characters like Min Min & Pyra/Mythra, who were too late for the initial game or FP1, got into this one, and Sephiroth got in to rectify the lack of content for the Final Fantasy series.
The only character that was planned way back then was Steve, who was being heavily negotiated for ever since 2015 but hadn't been able to be finalized until sometime during Fighters Pass 1, most likely, since otherwise he would have been included in the Base Game or as part of Fighters Pass 1.