Don't play Nasus, Jax or Talon top. Good players will spank you for doing-so. Wukong is similar to Riven, but I don't know of many good matchups for him, as his combo costs too much mana to spam and he needs to do the whole thing to stay healthy(has 0 potential sustain). Pantheon is like Riven, but he sucks top imo. Whenever I try to top with him I cry a little inside, 'cause I love Panth so much and wish he could fit in with the other beefy, manly top chars. Olaf is VERY good, but plays nothing like Riven. He's more of a sustained fighter than a bursty fighter and has no mobility whatsoever.
Here are my suggestions: Renekton, Jarvan or Rumble. All melee, all mobile, all bruisers/fighters.
Renek is most like Riven of the three with a duo-dash, a melee-range stun, a heal instead of a shield and a "steroid ult." I don't play him often because while he can dominate almost any opponent top, he falls-off later in the game. He can dive a lot like Riven and really does stomp most tops, though.
Rumble is second-most like Riven, with good mobility, harass that's hard to answer and a shield. Both he and Renek play like bullies, which I feel is Riven's role in-lane. However, rather than being a superb duelist, Rumble is better at poking until they can't fight back, pushing lanes and being the biggest headache of a teamfight. Rumble is the best of the 3 imo, because he offers your team the most in teamfights without losing the lane to too many people. Usually he'll lane alongside his lanemate, gaining slight advantage after lvl4. Not many bad matchups for him, but not a huge number of great ones either.
Jarvan is my favorite top solo. He isn't much like Riven when it comes to playstyle, but fills a similar bruiser role in-lane. He's got very good harass, the ability to be built tanky or tanky AD, LOTS of versatility build-wise(runes and items) and has the ability to deal the most without your opponent having a chance to deal you any in return(thanks to his init popup). He wrecks a lot of AD matchups thanks to his standard(Tryn, Fiora, GP, Nasus, Talon, Trundle, Panth, AD sion). He also offers a lot more to his team that Renek or Riven in fights, in-that he can give attack speed/armor, reduce armor by percentages and has 3 cc. Played well, he has many very good matchups and a few pretty bad ones. He tends to fall to Irelia and has trouble with Riven. That's bad because they're popular, but it's not the worst because they're his only really terrible matchups that are viable.
If I had to choose one for you to play, I'd choose Rumble. Jarvan is my favorite, but Rumble is the best of them. He has his own mechanics to learn, low cooldowns, good damage, offers your team the elusive AP top, good sustain through Will w/o mana and an absolutely awesome passive that makes you turn red and makes his mek "Tristy" blare "WARNING! WARNING! WARNING!" I really like his laugh, too. Yeah, go Rumble. Not enough Rumbles in the world. Oh, and people will underestimate you, since nobody plays him and he starts slow.