1. Alcuard: Even Sakurai originally considered Alucard over the Belmonts, and while he would definitely be a liked character among most players, it does make more sense to chose a Belmont as a fighter first to better represent the series. But anyway, Alucard's AT is already on the stronger, more versatile end, and he even has dialogue in Richter's Pit & Palutena chat. I wouldn't rule him out of becoming playable in a future Smash Bros title, or even in a theoretical Season 2 of DLC for SSBU. He has a lot of options as a fighter, although hopefully if he ever was included in the roster, he wouldn't become overpowered.
2. Shadow: At this point, I think the Sonic franchise could use another rep. It's been the most consistent third party franchise in Smash Bros to date, and at this point I don't even see Sonic going away from Smash unless relations between Nintendo & Sega to back to how they were in the 90's. If Shadow was playable, I hope they make him a semi-clone of Sonic at most, since at this point him and Sonic have diverged a lot as characters. I would make his B Special one of his weapons from Shadow the Hedgehog or Chaos Spear, his Side B the Fire Somersault from Sonic Adventure 2, and his Up B being Chaos Control as a sort of teleport that leaves behind a small time slowdown effect where he previous was (both him and Sonic share their Down B Spin Dash). Of course, I could see Shadow being a slightly slower, but stronger version of Sonic, which at first sounds far, until realizing that Sonic is so insanely fast compared to the rest of the cast. Sonic's run speed in Ultimate is 3.85, and even if Shadow's was 3.35, that would still be nearly a full point ahead of speed compared to the game's current fastest, Captain Falcon. A version of Sonic with actual projectiles and better kill potential could easily be broken, so hopefully like Alucard, if Shadow ever did upgrade to playable, he would be balanced, yet not an echo fighter.
3. Bomberman: Honestly one of the few Assist Trophy reveals that somewhat bummed me out. Everytime I summon Bomberman as an AT if I'm playing Classic mode or more casually with friends, I always can't help but think, "Yep, it's him, Bomberman, going around placing bombs everywhere. Really wish he was playable." Maybe it's asking too much of Konami to have three franchise reps since we currently already have Metal Gear & Castlevania. Super Smash Flash 2 showed how Bomberman could work, although hopefully if he was included in an actual Smash Bros game, he wouldn't be a rushdown character like in SSBF2, but more of a tactical bomb placer instead. Also Bomberman Heroes was my most played N64 growing up, so I do like the character and his franchise a lot too as a bias factor. He's iconic enough, and his many games in the franchise do show moveset potential, and I'd like to see him get promoted to playable one of these days.
4. Isaac: Just realized he's the first non-third party rep I've added to this wishlist lol. Anyway, I think Isaac as a character could add a lot to Smash Bros and he wouldn't be no generic sword fighter. In the Golden Sun games, Isaac uses Venus Psyenergy, making him use a lot of nature-based attacks like rock spires, thorny vines, tremors, and several other earthly techniques as well, which aesthetically pleases me since I'm a huge fan of earth-based powers. Sadly nothing is being done with the Golden Sun series for now, and I know he might seem too irrelevant for Nintendo and Sakurai to make a playable character for. His best time to have gotten into Smash Bros would've been during Melee, and unless Golden Sun gets another game again soon, I don't see him having a window of opportunity unless the Golden Sun fanbase starts getting extremely vocal and persistent for their efforts of getting him in. He has my support of becoming playable, although I wouldn't mind if he went with his Dark Dawn design since he older character would also fill in some niche roles as well.
5. Ray MK III: Oh boy, I know this one's definitely got a lot against him, especially since he hasn't even been an Assist Trophy since Brawl, and Nintendo hasn't even bothered renewing their trademark on the Custom Robo series. It's a shame really, I think a small robot character would be a nice fun addition for Smash Bros. A small gunner character, one could argue that he's now best represented through a Mii Gunner wearing a costume based off him. While this is true, the real deal I think would be a far more important figure to have added in Smash Bros. Sadly like Isaac, Ray MK III suffers from having a currently irrelevant franchise, and I don't see Nintendo doing anything with Custom Robo anytime soon, but a man can wish to see this lil' robot become playable one day.
Honorable Mentions
Akira - Good Virtua Fighter rep, would bolster his series.
Samurai Goroh - Would allow for F-Zero to finally get another character to represent its series.
Knuckles - If not Shadow, I wouldn't mind seeing him become playable either.
Rathalos - Would be absolutely hilarious, especially if he's not even scaled down that much. Perhaps Smash Bros could take some liberties and use a younger Rathalos as the fighter rep, but another dragon-based character is always cool.
Zero - Would also be a nice representative to Mega Man, and is a highly popular character in his franchise as well.