Definitely Goku. Their previous encounters in the Dragon Ball series make it a natural choice. Their power levels and different fighting styles would create great tension and action in the story.
Beerus could be an interesting addition. As a powerful god of destruction, his interaction with Broly, who has such wild and uncontrollable power, would be fascinating. There could be a power struggle or Beerus trying to tame Broly's power in some way.
John McClane from Die Hard would be a great addition. He has a similar tough - guy attitude and is good at handling high - pressure situations in urban environments. Another good character could be Ellen Ripley from Alien. Her experience with alien threats would contrast well with Rambo's combat skills. Also, Indiana Jones would be a perfect fit. His knowledge of archaeology and adventure - seeking nature combined with Rambo's combat expertise could lead to some exciting storylines.
Storm from the X - Men would be a great addition. She has control over the weather, which could create some really interesting scenarios when combined with Wolverine's combat skills. For example, she could create a lightning storm while Wolverine battles enemies in the rain, using the flashes of lightning to his advantage.
In a crossover with Hannah Montana, obviously Miley Stewart as Hannah Montana herself. She could be in Seattle for a secret concert and needs iCarly's help to promote it without revealing her identity. It would be fun to see Sam trying to keep Hannah's secret while also being her usual wild self around her.
In a crossover with the Lord of the Rings, Aragorn could be an interesting choice. He could bring his leadership skills and combat expertise to Los Santos. The GTA 5 characters might initially be skeptical of his noble ways, but they could learn from him. He could also help them fight against a new evil force that has emerged in the city, perhaps an ancient and powerful gang with dark magic - like abilities.
If it was a crossover with The Evil Within, Sebastian Castellanos would be a great character to include. His detective skills and his ability to handle the nightmarish creatures in his own game could be useful in the Outlast 2 world. He could team up with the Outlast 2 character to solve the mysteries behind the horror. Also, Pyramid Head from Silent Hill could be a terrifying addition. His presence alone would heighten the sense of dread in the Outlast 2 crossover story.
Well, if you're doing a crossover with '101 Dalmatians', Cruella de Vil could be an interesting addition. She might see the Aristocats as a new target for her fur coats, creating a whole new conflict for the cats to face. And the Dalmatians could be allies of the Aristocats, helping them outwit Cruella.
Goku from Dragon Ball is a must - include. He's the iconic Super Saiyan. And then maybe Thor from the Marvel Universe. Their different powers would make for an interesting dynamic. Also, Link from The Legend of Zelda could be a great addition, as he has his own set of unique abilities.
Characters from other music - centered shows like 'Glee' could be great. The members of Lemonade Mouth could interact with the Glee club members, have singing battles or collaborate on a big musical project. It would be interesting to see how their different styles of music and group dynamics play out.
In a crossover with 'Boardwalk Empire', Nucky Thompson would be a great addition. He and Tommy Shelby could have an intense power struggle or form an uneasy alliance. Another good character to include in a crossover would be James Bond. Imagine Bond infiltrating the Peaky Blinders' operations, thinking they are part of some international criminal syndicate.
For a crossover with the 'Hunter x Hunter' universe, Gon Freecss would be a good choice. Gon's natural talent and his growth throughout the 'Hunter x Hunter' series could be compared and contrasted with the leveling system in 'Solo Leveling'. They could go on an adventure together, exploring new lands and facing challenges that require both of their unique skills.