I guess I would turn the question around a bit and ask what, specifically, can the current class (and role) system, including multiclassing and the forthcoming PHB3 hybrid class rules, not achieve. Are there things we cannot accomplish with the current system, or that we cannot accomplish without a prohibitive cost?
Since the example of a fighter using ranged weapons has come up, I guess that is as good a place to start as any. I am not talking about the fighter using ranged weapons to fulfill his role as a defender -- just a fighter using ranged weapons in a useful way. I think the rules support this.
1. A fighter is proficient in military ranged weapons. With a good Dex (which many fighters will have), he can have a good ranged basic attack.
2. By virtue of his Combat Challenge ability, a fighter can mark an enemy and impose a -2 penalty to that enemy's attack rolls with a ranged basic attack. Ranged basic attack + Combat Challenge = 1[W] + Dex modifier damage on a hit; Effect: the target takes a -2 penalty to attack rolls (except against you) until the end of your next turn. That is honestly solid.
3. Multiclassing into ranger can give him access to 1 encounter and 1 daily power that works with ranged weapons (and might even work with melee weapons, if he chooses properly). For example, Attacks on the Run is awesome. Two attacks, 3[W]+mod, does not require two melee weapons, so works for any fighter.
4. Taking a paragon path from his multiclass can give the fighter access to 2 more powers that work at ranged (and if judiciously selected, might work for ranged and melee).
5. The fighter could go whole hog and do paragon multiclassing. Probably the weakest idea here, but I think it can work.
Here is an example level 4 fighter (so not even taking advantage of all the options are the paragon tier) that has tried to add some ranged attacks to his repertoire:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 4
Human, Fighter
Fighter Talents: Two-handed Weapon Talent
FINAL ABILITY SCORES
Str 18, Con 12, Dex 16, Int 10, Wis 12, Cha 8.
STARTING ABILITY SCORES
Str 16, Con 12, Dex 16, Int 10, Wis 12, Cha 8.
AC: 19 Fort: 19 Reflex: 16 Will: 14
HP: 45 Surges: 10 Surge Value: 11
TRAINED SKILLS
Perception +8, Streetwise +6, Heal +8, Athletics +11, Endurance +8
UNTRAINED SKILLS
Acrobatics +5, Arcana +2, Bluff +1, Diplomacy +1, Dungeoneering +3, History +2, Insight +3, Intimidate +1, Nature +3, Religion +2, Stealth +5, Thievery +5
FEATS
Human: Warrior of the Wild
Level 1: Sideways Defense
Level 2: Quick Draw
Level 4: Novice Power
POWERS
Bonus At-Will Power: Reaping Strike
Fighter at-will 1: Footwork Lure
Fighter at-will 1: Cleave
Fighter encounter 1: Steel Serpent Strike
Fighter daily 1: Flanking Assault
Fighter utility 2: Defensive Stance
Fighter encounter 3: Sweeping Blow (retrained to Disruptive Strike at Novice Power)
ITEMS
Scale Armor, Greatsword, Longbow
=========================================================
This fighter's ranged basic attack is only 2 pts behind his melee attacks (1 for proficiency bonus, 1 because of higher Strength). With Sideways Defense, he can mark an enemy at range and then stand adjacent to his allies, giving them a bonus to defense (in additional the effective bonus from marking). If someone does decide to attack his allies, once per encounter he can use Disruptive Strike to interfere with their attack -- and this works not only with the longbow, but with the greatsword as well. It is actually a very defender-like power that happens to come from the ranger class.