Dragonball Evolution failed so much with critics and audiences -- especially fans of Dragon Ball Z -- that any consideration of a sequel was dashed, despite the film's end alluding to the saga continuing and plans for a follow up film formerly in the works. The movie loosely adapted the "Demon King Piccolo" arc, while bringing in elements from the first arc of Dragon Ball into the mix, so it is logical to assume that the next step would've been to adapt the "Saiyan Saga;" however, when analyzing the messy way Dragonball Evolution adapted Dragon Ball, it becomes clear that, while the "Saiyan Saga" may have been adapted in loose terms, a sequel would have resulted in a film arguably worse than the first movie.
Could the Saiyan Saga be Adapted in a Dragonball Evolution Sequel?
When looking at the elements of the "Saiyan Saga" that could've been adapted for a sequel, there isn't much that would've made it. The Saiyans could play a role, and Vegeta would probably be the big bad; meanwhile, Goku would have to work with Piccolo.
Piccolo for sure would have undergone his redemption arc in this film, if comments by James Marsters are anything to go by. Marsters teased that a sequel to Dragonball Evolution would've focused on Piccolo becoming young again and becoming the character fans are more familiar with.
Some components of Dragon Ball that weren't adapted into Dragonball Evolution might appear as well, like Krillin, who was absent in the film. Goku could've also been killed and restored by the Dragon Balls; however, these might have been the only things that could survive in a sequel.
Gohan May Not Have Played a Role In a Dragonball Evolution Sequel
Arguably the biggest change would be a lack of Gohan. While Justin Chatwin, the actor who played Goku in Dragonball Evolution, teased the possibility of new characters appearing in a sequel, it seems unlikely Gohan was one of them.
By the end of the first film, Goku and Chi-Chi aren't officially in a relationship, and they are both high school students. Ergo, it would be quite the stretch to show them with a kid by the next movie, most likely requiring a time-skip. Furthermore, it's likely they'd keep Gohan as a four-year-old, so he'd have a reduced role, if any. If Gohan had no role, then that prevents a large amount of Dragon Ball Z from being incorporated into the film, and even things teased, like Piccolo's redemption, would have to be changed.
Undeniably, Gohan would have to be altered drastically in order to fit into the continuity established. Dragonball Evolution took no risks adapting its fantastic source material, choosing to dumb down the fantasy elements until they were nonexistent, so a rage-powered four-year-old who turns into a giant ape would likely be out of the question.
Saiyans Could Be Different in a Dragonball Evolution Sequel
In Dragonball Evolution, Oozaru -- the Great Ape transformation that Saiyans undergo -- was the servant of King Piccolo, indicating that Piccolo, after invading Earth, took a Saiyan slave that ultimately turned out to be Goku. This indicates that the very reason why Goku arrived on Earth is different from the original series.
The problem here is Piccolo was sealed away 2000 years prior to the events of Dragonball Evolution, which indicates that Goku was sent to Earth thousands of years before the events of the film. In the original series, by contrast, Goku was sent to Earth right before the start of Dragon Ball from Planet Vegeta, which had just blown up, so it's unclear what the state of Planet Vegeta and the Saiyans are before film Goku's departure. Furthermore, it's unclear if the Saiyans have an alliance with Piccolo, let alone how Frieza fits into all of this.
This means that, regardless of any changes, if other Saiyans were introduced into this theoretical Dragonball Evolution sequel, they would be unlike anything previously seen, potentially being different characters all together. Much like the rest of the Dragonball Evolution continuity, the lore would be so unlike Dragon Ball, it could feel inaccurate to call it an adaptation at all.