It is a fickle line to walk with Marcus as a long standing Trilogy hero and icon for the franchise. At the same time, just like Halo, there is a world out there that doesn't involve the singular exploits of Marcus. Tons of military gears units around the world were operating at the same time he and Dom were saving the planet's collective bacon. The elements that made Marcus great was his charisma and the balance of other characters supporting the storytelling (Cole, Tai, Dizzy, Anya etc.)
Tie that in with the link between his father and the research behind a solution to the locust problem and you discover he was at the core of it all and was the focal point of interest. Despite being gruff, scarred up, emotionally detached and nearly impossible to get into a public relationship with Anya, we were drawn to him for his kick-butt personal demeanor and gravely voice. Personally I think if they make a new game they should go back in time to the days when Marcus' father served as a gear to steal the Hammer of Dawn Technology during the war with Garazni. Having Adam Fenix, while not as violent and action charged as his son, was still integrated with tons of great characters that could create that similar arc and tie in with winning the war.
Do we need the Locust? If we do, Marcus still did around 10-14 years of service before Gears 1 even started... why not tell that story as it falls right in line with the timeline for where Judgement was. If you can't go forward cause the problem was solved (locust and lambent) go back wards or along a linear timeline with brand new characters who were contributing to the effort.