whats been trendy over the years as far as looks is just a renewal of old styles usually from 10-20 years ago. something thats beginning to come back is bringing back classic cup-sole shoes that everyone liked (the boss, k-1, staple, emerica mj, manchester i think, es arcane, dc biltmore). these are shoes that probably should have never been stopped being produced, so i'm all for this movement.
as far as skateboarding goes, hopefully there are fewer grind-grind combos and more fast and gnarly skateboarders like busenitz, andrew allen, alex olsen etc... i also wouldn't mind seeing a resurgence of good vert skating. by good i mean big steezy airs and smooth maneuvering on a ramp that looks good. i dislike watching jiggy switch flip grabs on some so-cal vert ramp at the YMCA.
lastly, above all that i have mentioned, the alien workshop video will decide what's trendy for the next 3 to 5 years if there's anything unique in it, which there most certainly will be.