Major issue here is I have not had any concept really pop into my head and I usually used that to make characters when not filling in for a role (been doing that since like 1987 let's have the 4 yr old play a cleric).
Here is the story a friend of mine is running a game with some people she met at local gaming meetup (many moons ago), I've met most of them they are interesting people and I've been to a few non-gaming events and enjoyed their company so that isn't really the issue. As with many groups they have complicated schedules and sometimes it is difficult to get enough people to game.
So another friend and I have been invited to join this game It is a 4th level game at the moment. That friend brought up an interesting point as it would seem that with our schedules being the least changing we along with the DM's boyfriend would be the player base with the a medium cast (bolded in characters) of guest stars. But here is the original cast if that helps:
Dragonborn Inspiring Warlord: He'll be at all the session. One of my best friends from college.
Human Fighter: Was built in the style of a barbarian before PHB2 was released and he doesn't really feel like switching the class. The player is from the 1st and 2nd editions with minimum exposure to 3rd so he isn't a tactical factor they got him using his at-will more effectivly lately. He's a nice guy so is general
Tiefling Warlock Fey pact: Soild player his wife is plays as well as something to do.
Tiefling Rogue: Foremetioned wife she is a social player, but has learned the basics.
Elf Archer Ranger: New to the group I think this is her first 4th ed game, she's a good player in other games, so isn't an issue.
Dragonborn paladin: His character was dominated and captured in a previous session so I'm not sure what will be going on with him in the furture.
New characters coming in:
Stormsoul Genasi Assault Swordmage
Half-elf Bard: Bow focused
And Whatever I come up with >.<
So looking for a bit of help on that front. Game will be starting up again in about 4 days or so.
Thanks for reading my walls of text.
Offhand, if you wanted to fill out the party you could make a controller, since they have two leaders, three strikers, and three defenders. On another hand, however, the bard and feylock will provide a lot of controllerish power, so that leads to a better option.
What do YOU like to play?
Currently the party has four martial characters, three arcane, and one divine. Two dragonborn, two tieflings, one elf, one human, one half-elf, one genasi. That means no primal characters, nor clerics, invokers, avengers, or wizards, sorcerers, or artificers. That leaves you with plenty of options, both for race and occupation, if you want to be something different, but also plenty of variation if you want a similar setup. What kind of character do you generally favor, and what playstyle do you prefer?
If you want to pair up with the paladin, for example, you could play an Invoker or Avenger of a similar god, perhaps using the Deva as a nice racial twist.
Or the party could use a resident magical expert, a wizard vaguely connected with the genasi or warlock.
Perhaps you want to use one of the current classes in a different build, say a human tactical warlord to synergize with the dragonborn, or a melee ranger with a beast companion in the thick of battle.
On the other hand, if you want to diversify the group even further to cover everything, consider bringing in a primal character to give them that natural touch. A druid might mix well with the feylock or the ranger as a friend and guide, or a barbarian could fraternize well with the warlord and fighter for furious melee capabilities.
Without knowing the exact details, and more importantly, what you actually like to play, it's a bit tough. My advice is to look over the list of classes outside of the current group again and see if anything appeals to you. If nothing immediately strikes your fancy, try challenging yourself. Pick one of them at random, read over its power list, and think about what might be a nifty core component of your character. Some classes have amazing and flavorful abilities, utility powers especially, which can provide an entire character concept.
Also think about the possibility of reflavoring. A warden isn't just a natural protector, he could also be a seedling treant, a wandering werewolf, an ancient golem. An avenger makes a great wuxia swordsaint, just as a summoning invoker does a great job as an insidious necromancer, especially if you take some of the Divine Power abilities that allow you to hurt yourself to harm the opponent even further.
If you need further ideas, tell us a little bit more about what you've enjoyed playing in the past, and think very generally about a concept or three you'd like to play in the future. Need a little more detail to help out much further, as playstyles vary quite a bit.
And if all else fails, play a Kobold Kommando. 
And if all else fails, play a Kobold Kommando.
This.
A thousand times THIS!
So, anyways, I'd recommand a striker or a controller/defender who goes into the striker path, you seem to have a couple of leaders, defenders and strikers, but no striker is bolded, so your encounters might be a little long without one.
My take would be a pursuit avenger, use overwhelming strike on a target marked by the swordmage and win.
Personally I recomend either Warden or Avenger. In addition to being sexy, they both can make a pretty effective team with the rogue (getting them combat advantage, and in the warden's case, drawing the baddies attention away from them).
My vote is also for the kobold. Every party should have one. 
The interesting thing about your group dynamic is that you have 3 regulars and large group of people who may or may not be there. Since the other 2 regular players are defender + Leader I would stay away from those roles, since if one of the other Defender/Leaders turn up, you end up with 3 of the same and one of you ends up being redundant.
Your regulars are melee based, but quite alot of the extra players are ranged, so in order to get your bases covered/sinergise with the rest of the group, you want a versatile character, both with respect to melee/ranged and also between your groups weakest roles, Controller and Striker.
Assuming you dont want to duplicate any class which may be there, you still have a broad range of options.
Druid, Invoker, Wizard, Sorcerer, Avenger.
Avenger I would rule out fairly quickly since they have little controlling ability and less ranged.
Wizard is perfectly viable, but given the Sorcerer has better close range ability and the invoker has the advantage of being divine, something that you party is also lacking, i feel those 2 make better choices.
My top 3 picks would be a predator Druid, melee (Acid orb+reaper's touch) Sorc or an Invoker (focusing on close atacks).
Choice of race would be more about which way you want to take the class and how you want to RP it, so I dont really think thats worth discussing here.
I always advocate that one should simply play whatever type of character they want.
So, what inerests you? Or better yet, what DON'T you want to play. Make a list & cross things off you don't want/like. Choose from what remains.
Things I'd consider long before I worried about what other people were playing:
1) Does your character have any connection with another character - either one of the originals or the new ones?
2) Where is this game set? Are their any choices that the setting influances? For instance, a game I joined a while back was set in a very Norse type region. Thus I had to have somethig that fit - or a really good reason for something different.
3) Story elements already in place. Is there something there that you should be considering?