Well, I guess it depends. Within my 'essential' third parties, maybe like 5-6 of them, I'm not willing to be very flexible. Frankly I don't think "zero new third parties" is a realistic option either given the current state of the series and expectations, so I would at least make the effort to carve out space for two or three of them. I guess I can't really give you a good answer unless I know who this anchor third party is.
How big is your roster? I thought it was a good size before, so adding a whole other line feels... a bit unnecessary (why do you need a whole new line instead of just reorganizing what you have?), but also probably overkill.
Last time it was 64 slots, I cut down on third parties so now it’s 56 slots. I’m really trying to make it as square as possible, I could just black out some of the slots on the last line though. The third parties I have right now are:
Pac-Man, Insanely iconic and also kind of invented the concept of a video game character.
Hero, From the series that defined what is considered an rpg. Also one of the best represented third parties in Smash.
Ryu, From the series that defined fighting games. Also pretty much Mr. Crossover.
Sonic, One of the most popular and recognizable video game characters ever. Mario and Sonic at the Olympics.
Steve, From the best selling game of all time.
The only third party character I feel super confident about doing next to those is the Tamagotchi/Digimon character (Cause the other ones I had done, Rayman isn’t famous or important enough, Capcom already has a character. (Tamagotchi is from the Bandai side by the way) And the only other idea I was really considering was some kind of indie game representative.)
Doomguy maybe?
As far as cut table slots the only ones I have that I feel like are cuttable are Dark Pit and DJ Octavio. Dark Pit is my slot to cut for if K. Rool comes back in Bananza. And I kind of don’t want to cut DJ Octavio because I’m imagining him being mechanically neat, And I don’t want to cut him for someone who wouldn’t be as mechanically interesting (Doomguy.)
So I’m really leaning to that Tamagotchi/Digimon rep even though it’s kind of an unconventional choice.