Maybe Top 30 all-time in both passing and rushing yards.
I don't really have much of an opinion on this but figured I would look into how far away Lamar is from these milestones.
He currently has amassed 15,887 passing yards (which ranks 151st all time). The #30 QB is currently at 36,638. However, there are several current QBs ahead of Jackson that will likely push the bar higher for the 30th spot (Tannehill, Goff, Prescott, Mahomes, and Allen as examples). Let's project the Top 30 by then will be 40,000 yards. That would mean Lamar would need 24,113 additional passing yards. At his current pace, Jackson would need to play 15 total seasons to hit that number . . . which seems way in the future given that he's only played 6 seasons so far.
Rushing wise, he has 5,258 rushing yard (which ranks 137th). The #30 player in rushing yards sits at 10,273. But Derrick Henry and Ezekiel Elliot are close to there (and some other current backs could still make it). Let's say 11,000 will be the new Top 30 cut line in the future. Jackson would need to double his currently total and a little extra to get there. I'm not sure at what age he will start rushing less frequently, but I don't think 150+ carries a year will extend his career any.
I guess being a dual Top 30 threat is a possibility, but w are many years away from that being something to consider a lot more seriously. He has a lot of ground to cover to get there (although it is in the realm of possibility).