Realistically speaking, the biggest hurdle that Captain Price and Call of Duty faces is mainly the fact that it's too realistic IMO. The very nature of Call of Duty clashes hard with what Smash has, and although there are other series represented within the game that also deal with very mature themes - Metal Gear being the closest, they at least all have some form of fantasy-related chicanery to offset that.
(Plus, let's all be honest: Call of Duty and Smash have to stay away from each other. That isn't happening.)
Also, I seriously doubt they'd retire Heihachi. If they really wanted to make a quick buck off him as DLC, wouldn't be that surprising to just recycle Unsho Ishizuka's voice clips from Tekken 7. If they want a Tekken rep but have to stay away from him for whatever reason they have, Jin and Kazuya would be serviceable alternate choices.
The theoretical second Bamco rep really just boils down to Tales and Tekken for possible choices, with Soulcalibur being a distant third. Lloyd and Heihachi were already Mii Costumes in Smash 4, and a good amount of those characters (Chrom, K. Rool, Isabelle) did get promoted to the main roster. The other Mishimas can rival Heihachi's significance enough to serve as back-up picks, while Yuri's recent prominence due to the Vesperia rerelease could help his chances too.
Can't help but feel kinda melancholic though about learning who the next two fighters are in the near future. Smash Ultimate hasn't had the best track record with trying to keep things secret. Only a handful of characters (Richter, Plant, Joker among others) survived not getting leaked or having their names thrown around a lot within the speculation scene, and I feel like we only really need to wait 2+ months or so until some new insider hints at "X" being DLC 4 or 5.