Wow, an interesting thread has been revived from long ago. I was rather surprised to see an old post of mine that I vaguely remembered when I was reading through this thread. Its amazing how my thoughts upon reading through this thread this time were rather different than those from a few years ago (though my opinion that a 4e Modern game would be awesome still stands).
I like a lot of the ideas that were tossed around back then, like having a gestalt system that let players freely link combat roles with non-combat abilities, or implementing the tier system in a way that resets the level count at each tier.
Talking about roles in a modern game has gotten me thinking. If you want to create a role-based system for a modern game, you may very well need to create very different roles than what you see in 4e D&D. A large inspiration for me in this regard is the videogame Valkyria Chronicles, which is an incredibly good tactical RPG set in a pseudo-WW2 setting. That game only has six classes that all fulfill very different purposes, making it into a very interesting study on how to use classes and roles in a game where everyone uses guns. In that game, which is heavily built around the idea of interception fire, classes with high defense (Shocktroopers, Lancers, and Tanks) are the best attackers, while the best defending classes (Scouts and Engineers) tended to be fragile and mobile. At the same time, there are few real absolute distinctions between offensive and defensive classes in that game, since almost any class can be used for either depending on the situation. And there are also classes like the Sniper, which have no mobility, no defense, and can't use interception fire, but are basically guaranteed to kill any opponent who isn't standing behind cover. You really can't describe these classes as 4e Strikers, Defenders, Controllers, or Leaders, but they nonetheless function in clear roles that work quite well.
Based on what I have seen in some of the Valkyria Chronicles classes, I came up with the following roles that might work in a 4e Modern game:
Vanguard: Tough fighters who are designed to charge through enemy interception fire and take down enemies in close quarters combat. They combine high defense with high offense, but they simply lack the mobility of some other classes, and don't have a lot of range. Machine-gunners and melee fighters would both fit into this category. They are better described as short distance sprinters than as endurance runners. Their biggest specialty is taking out opponents hiding behind cover. Not very good at interception fire, but are great at holding guarded choke-points. Good against most types of enemies. The vehicle equivalent would be an armored tank.
Ambushers: Agile fighters who emphasize mobility and range. They don't have a lot of defense or attack power, but they can lay down excellent interception fire and can protect wide areas. They can easily die if they run into an area of enemy interception fire, but they excel at sneaking around the periphery of such areas and striking at enemy vulnerabilities. They more resemble light endurance runners than strong sprinters. They are bad at attacking enemies under cover, but are good at getting around that cover or forcing enemies away from cover. Is great at fighting multiple weak opponents. The vehicle equivalent would be an armed car or helicopter.
Snipers: Skilled masters of weapons who can take down opponents with great accuracy. They have bad mobility and can't take many hits, but have unmatched destructive power. Are generally ineffective against enemies under cover, and have no ways to easily get around enemy cover. Is the best at fighting single strong opponents. The vehicle equivalent would be an artillery piece.
Support: Engineers, medics, and team leaders. Excellent an enabling their allies to achieve greater things, and helping them when they get into trouble. Average-to-low attack, defense, mobility, range, and interception ability. May have a few potent tricks up their sleeve.
I have a few thoughts for non-combat roles, but I'll save that for later.