I say it's a toss-up between 109 titles and 2 Calendar Grand Slams. Probably a bit more so the latter, considering the timing required. Laver's feat was truly remarkable and may never be achieved again.
At the rate Roger's going, I can see him passing Pete's 14 slams, providing he stays healthy.
The "easiest" record for him to break would probably be Andre's 17 Masters shields, seeing as how there are 9 of those tournies each year and Roger can grab at least one of the March double, one on clay, and any of the summer or fall ones every year, and he already has 10.