Based on the heroes you have now, work on Gizmo, Leah, Garrick, Riley, and Ezio. Keep in mind you are also lacking a good tank. Both Ezio and Riley are front line DPS heroes. As soon as they're down, your opponents have an open shot to kill the rest of your team. So work on Drago right away; he's a great tank—if not the best. I also find Leon very useful, but make sure you work on Drago first then Leon if you'd like. Also, work on Jasmine right away when you get her. She's very useful. I use Jasmine as a support and a healer, not a DPS. That is why I only have her at 3*; she gains her ultimate much faster that way, and she's amazing at disrupting enemy ultimates in the CoF and great for countering physical heroes for her blinding effect.
Other Suggestions: Riley isn't as useful at higher levels in the Arena. So if the Arena is the most important aspect of the game for you, wait on her—but, Riley is still very useful in Raids. She is also useful in the CoF, though she is not in my CoF team.
Ariel shines at lower levels, but her power diminishes later on. You can work on her until you hit around level 70+ if you have the time and money. If not, drop her.
Leah and Gizmo work best at higher levels. Gizmo deals a lot more damage when he's in orange fusion and his gear is fully enchanted—same goes for Leah. Both of these heroes I began using at higher levels and I highly recommend.
I recommend you keep working on Cara. She's not crucial at your level, but she's a must have later on, in my opinion. She's great at countering physical heroes (as expected), she's great at countering tanks, and she's an especially great counter against Ezio, Garrick, and Riley. She also makes the CoF a lot easier and faster for me. At orange fusion, she gains +400 energy after every battle, so her ability is almost always or close to ready for the next battle. Once the tanks are down, everything is smooth and easy. Also, I disagree with Mochokara; Cara's fourth ability is very much useful. Her Nighmare ability has saved my team a lot of times