I think the best question might be what if he continued to be an avid racer as well as a filmmaker. Knowing that NASCAR was still very much a 'Southern' sport in the 1970s I'd wager that if Lucas is gonna invest his time, money and interest into car racing he'd probably go for IMSA or Indycar, probably the former because there was a lot of guys in IMSA at the time who made names for themselves as both drivers and team owners - Rob Dyson, Al Holbert, Bruce Leven, Irv Hoerr, Jack Roush, Dan Gurney, Jim Trueman, Bob Tullius and Bob Garretson among the ones with a more positive impact. (The less said about Randy Lanier, John Paul Sr. and the Whittingtons, the better.)
So, flush with cash from the success of American Graffiti and Star Wars, Lucas goes racing in IMSA just as the GTP era opens. He first enters into the GT categories, and with the arrival of the Porsche 962 in 1984 he moves into the big game, joining Holbert, Dyson and Leven (among others) with Porsche prototypes. Wins a bunch of races along the way, and gets all kinds of attention for being the Hollywood guy. After IMSA founder John Bishop has heart surgery in 1987 and sells IMSA, Lucas is one of the buyers and gets the job of running the series' PR and image departments. He excels at this, and while IMSA's level of competition struggles with the entry of Nissan in the late 1980s, it's image stays high. Following the end of the GTP era, Lucas is one of those who runs in the WSC category, probably getting IMSA to allow turbocharged WSCs and thus being one of those with the Porsche WSC95. The WSC95 and rivals with Ferrari 333SP, Riley and Scott Mark III, Lola B96/10 and Courage C52 race cars with a variety of powerplants prove to be just as capable as the GT1s that come to dominate in Europe. By 1997, the profile of IMSA has drawn factory GT1 entrants from McLaren, Panoz and Porsche, with Mercedes eventually to follow.
Lucas' team is several times class champs and he gets his biggest victory as a team owner by winning both the Rolex 24 at Daytona and the 12 Hours of Sebring in 1995 with the Porsche WSC95. IMSA's popularity with participants rebounds massively in the 1990s with the commercially-available and very fast WSCs, helped by the Ford (through Riley and Scott and team owners Rob Dyson and Wayne Taylor) vs. Ferrari vs. Porsche battles and ever-faster GT cars. Rules are deliberately kept wide open to allow participants to push their way into the sport, and lots in the 1990s do just that.
Financial troubles for multiple individual series owners sees IMSA ultimately sold to Lucas, Dr. Don Panoz, Rob Dyson, Jack Roush and Bruce Leven in 1998. The massive crash of the GT1 category and ever-faster LMP1 cars thanks to factory involvement from Audi, Cadillac and Chrysler in the early 2000s leads to rules changes to allow newer chassis like the Riley and Scott Mark IIIC, Lola B2K/10, Courage C60, Dome S101 and MG-Lola EX257 to be competitive. Panoz and BMW stay in the prototype category, helped by BMW's exceedingly-lucky 2000 season where weather, lap car screwups and circumstances hand them two race wins on a platter. Lucas steps back from his team ownership some in the 2000s to focus on promotion of the sport. He does well at this, and Lucas' get teams make a big bet on buying the dominant Audi R8 chassis in 2001, and they (along with the Champion team and the factory R8s) run away with the 2001 IMSA American Sports Car Championship. Lucas gets the biggest win of his team's career in 2005, when his team with their now-aging R8s are faced off against faster Pescarolo-Courage C60s, Dome S101.5s and factory Cadillac and Chrysler entries to win the 24 Hours of Le Mans.