Given all the size of today's contracts and a number of other factors, I'm not sure if we'll ever see another athlete play in both MLB and the NFL at the same time. Fewer than 70 men have played in both sports and only three (Brian Jordan, Bo Jackson, Deion Sanders) managed 300+ MLB games and 30+ NFL games, roughly two seasons worth in each league.
Here's a clip from July 1990 featuring two of those players: Sanders and Jackson. Sanders, then with the Yankees, hit an inside-the-park home run past a diving Jackson, who was still with the Royals:
Man, so much speed and athleticism. Both of them.