While getting and investing in a some new lenses (ones that are faster due to a wider aperture; a wide-angle and telephoto to have their unique perspectives, advantages, and disadvantages; possibly choosing fixed length lenses over zoom) and other equipment is a good idea for the long-run, when starting out, it can be advantageous to keep it simple. Are you already good at metering and focusing, or will you shoot mainly in automatic modes? If you plan to do most everything manually, and you still need to practice those skills, having to do that and choose to switch between multiple lenses may just make things more confusing. And while changing lenses, you may miss any spontaneous action that can take place at a live event (I'm not big in the con scene, so I don't know how rowdy it can get). The same thing applies to having to think about focusing and metering in fast-paced situations--even professionals will use a camera's automatic features when timing is critical.
About the focal length of the lens: you need to know what the basic 3 categories are good for or else you're just blindly using them and not choosing them for their applications. What you have now gives you perspective about in the range of the human eye. Not a bad thing. Most pictures you'll take will have less distortion in relation to how we humans normally view things. When you use a telephoto lens, depth of field appears compressed, and it seems deeper with wide angle lenses. Both should be used for stylistic and technical reasons. Any good kit should have one of each in my opinion. But build your kit as your budget and experience permit. You also will need to decide if you want fixed lengths or zooms. A lot of people (especially ones who want to think they know what they're talking about) will deride zooms and swear by fixed (or only recommend them). The truth is, zooms have very practical applications. They also give you a variety lengths without having to change yours lens when moving forward or back isn't an option (these situations do exist!). The problem is that you sacrifice quality--more so when the zoom covers a broad selections of lengths. Many times, and with good photographers, the quality doesn't suffer. They know how to work around the limitations of their equipment and take advantage of what their tools offer. Forget pro or not, what matters is if your photos look good. Do you like them? Too many people get stuck in superficial technicalities without really understanding how they effect aesthetics. If you don't know jack about composition, color, and showing something worth seeing, then having expensive stuff won't change that.
In my opinion, as someone who takes lots of photos in the field, I don't want to be bogged down by too much weight and bulky stuff to carry. If you can't justify using it, you shouldn't take it along. When learning to master your equipment, it may not make sense to tote around all this stuff you haven't worked with yet.
A detachable flash is great for any kit, but I wouldn't recommend getting one till you know you understand the basics of light. For all I know, you already know a lot about of lighting. Light, of course, effects your ability to represent what is happening. Without adequate lighting, you can't take a photograph. That's why everyone is stressing the wide aperture, so you can have a fast lens that captures in-focus photos under lower lighting conditions. Also important with lighting is the color temperature of the light. Will you like the color that fluorescent lighting gives you? Or will you want to change the white balance? When you take many photos without a flash and see what you can and can't capture, you'll start to build a foundation to understand why flash can be good for future use.
There are other things to consider for your kit as well. What about a tripod? Having something to steady your camera can compensate for a smaller aperture. But good tripods and heads are expensive. And they can be bulky as well. Just using a basic one (if you have no problem bringing it along or into the convention) might be worth while. Or it can be cumbersome. Get a polarizer if you don't already have one. And if you don't have a uv filter either, get one. A lot of people keep them on their lenses at all times for the added protection from scratches. Like another poster said, having more cf cards and battery is always a good idea.
My suggestion is to consider bringing minimal equipment along and seeing what you can and can't capture/represent as you desire to give you an idea of what you will need to buy to get the results you want in the future. There's no reason why you can't just use the kit lens for now.