Pretty much any actors and actresses that we consider giants of the big screen will go on to be legendary in the future, I suspect. I think certain ones have already hit that status of being unforgettable, like Tom Hanks, Harrison Ford, Carrie Fisher, Robin Williams, Meyrl Streep, Sigourney Weaver, Jamie Lee Curtis, Sean Connery, Drew Berrymore, etc... These people aren't really modern actors though, they have had 40 or more years in the industry, some are even still acting.
I would say most actors and actresses who started in the late 80's and into the 90's, only having about 20-30 years in the industry are modern enough, yet have the track record to reasonably deduce whether they will be considered timeless actors.
I'd say Scarlett Johanson, Sandra Bullock, Tom Cruise, Keanu Reeves, Brad Pitt, Natalie Portman, Will Smith, Matt Damon, Russell Crow, Nicholas Cage, Jim Carrey, Angelina Jolie, and Leonardo Dicaprio are some who will go on to be very well renowned even in the future, and there are more that I haven't even listed.
Robert Downey Jr. started acting in the 80's, but his popularity didn't really begin to grow until the last 20 years, I think he's set up a good chance for himself to be considred one of the greats and I'd consider him modern.
At one time I would have said Shia Lebouf, hands down, and I suppose the legacy in film that he has already left behind is enough to potentially make him timeless, but then he went off the rails and ruined it for himself, fading away from big roles for awhile. Maybe he'll make a comeback and stay grounded, who knows.
As for up and coming actors and actresses, Jennifer Lawrence, Zoe Saldana, Chris Pratt, Zendaya, James McAvoy, Ryan Reynolds, and Tom Holland are a few that come to mind that have a lot of potential and are in really good positions to be sought after for other big roles in the future that could forge them a way into history as memorable actors and actresses.
But in the end, it's more about the movies themselves and not the actors and actresses that make it timeless. A movie could have an amazing cast but still turn out bad for some reason. If a movie is timeless it had good writing to begin with, that's when the other factors come into play, like the performance of the actors and actresses, the special effects, and music, all accompanying the writing to make something that stands the test of time.