They'll have to come up with something, since there isn't an especially strong relationship between Bruce and Slade in the source material--but it'll never be quite like Dick and Slade. There's so much history there, much of it very personal.
I think Dick Grayson (and more broadly, Robin) is one of the great missed opportunities of of the DCEU. Robin would have provided such a fresh perspective on Batman (singlehandedly justifying the reboot), and Dick is, in many ways, a lynchpin of the DC Universe. He's one of the most human and instantly likable figures, a relatable gateway for the audience, but he's also tied intrinsically to so many heroes and villains by virtue of having been raised in that environment.
While Dick could appear in the DCEU, it'll only be at a point in his life where so many of those great storytelling opportunities have already been missed.
Personally, in a cinematic universe starting from scratch, I think Dick could have been the Tony Stark of the DCEU--a gateway to the rest of the universe. A Batman film in which Grayson is the main character, and we see his origin story through his own eyes with Batman as his mentor, I think really would have set the stage perfectly. Through Grayson you can be re-introduced to figures the audience has seen a million times, but from a completely new perspective.
It will never be, but I can dream. In the mean time I'll have to hope they introduce Tim Drake to help reel in the DCEU's dangerously unhinged Batman.