A couple suggestions I've learned the hard way... don't use extra lean ground beef when grilling because it tends to dry out; especially as leftovers. Fresh herbs are always better when possible. Have everything prepared before you start because the meat cooks so quickly you wouldn't want it to cool off if everything else isn't ready. You can add all sorts of toppings: fancy Dijon mustards or mayo spreads, various cheeses, pickled yummy things, sprouts, cilantro or other baby greens. I like the subtle taste of arugula and find it doesn't compete with the unique taste of the truffle oil.
Try to find a fat baguette, so that your slices will be more round than oval.
If you trust your meat source and have a fabulous chuck instead of ground grocery store beef, then grill your sliders to a perfect medium-rare/medium. Well-done if not.