Otto/Reggie is hard to decide, due to Reggie having an edge from the concept art but Otto having an edge fromt he show itself (usually the most front and center in ads and was always the one in the spotlight in the show itself; though if I were choosing I'd add both as echoes of each other).
I don't see what puts Ember off the table (unless you're talking about the Randy Stair stuff, which most people don't even know about), especially since she's in the Top 25 and the second most requested Danny Phantom character after Vlad.
As someone who's part of The Loud House's fandom I can give some insight in regards to Luna and any other potential reps. While Luna's the most popular sister, I've seen equal demand for Luan as well (and both have precedent from the Super Brawl games). There's also Clyde, who a lot of people tend to forget about. He's been in every crossover TLH has been in alongside Lincoln and a sister (usually Lucy) and was also in the Super Brawl games, so I can see him being chosen as well. Nick's marketing really seems to like him. In terms of NASB fandom popularity, Luna has a slight edge but not by much, given that she placed 115th on the popularity poll and was the highest-ranked Loud House choice. Some even cite Lynn Jr., and while being a stage cameo doesn't outright deconfirm the sporty sibling, she has two layers of polarization to deal with (from being a Loud House character and being extremely polarizing within the fandom itself). Ronnie Anne's all but screwed, given her spinoff's extremely negative reputation outside of TLH's fanbase, along with said spinoff having been cancelled last year.