There are a couple of archetypes that I think are long overdue for their time as a "core" class. They've pretty much all been mentioned already:
The Warrior-who-does-magic-too (Swordmage, Bladesinger, Spellsword, whatever) based off of the "Ftr/MU" seems to have the longest history (going back to the "elf" in B/X-BECM) with the game. So, if I got to pick ONE, as the OP asks, in the interest of "fairness" (to a fictional fantasy archetypal concept with no actual feelings

) and seniority, this would seem to be the winner.
The other "long overdo" archetype I would like seeing would be the "divine-arcane caster", Thaumaturgist/Thuerge/whatever they get called. A "Cle/MU" base class with access to Arcane and Divine magic lists, skilled with ritual magic, tons of knowledge skills, though I would keep the armor and weapons light if not non-existant. Cover the "holy-minded/spiritiually-devout wizard" and/or the "occult-delving clergyman/cleric."
Also, I do think Shaman (in the archetypal sense of one who deals with totems, speaking to and traveling in the Spirit world) fills a nice opening. I've never considered Druid's to be very "spirit world dealing guys."
Personal favorite would be a decent flavorful archetypal Witch class.
But I can also get totally behind Yora's suggestion that it could/should be something "non-magic-castery." I think the more limiting the spell-caster classes are at "core" the better, and sets a nice (unspoken) tone for the game as a whole.
In this case, the Scout is good, but I think a bit tooooo close to Rangery for me to justify a whole separate archetype (which is something I think something that is going to be a Class, in and of itself, needs). I'd probably vote for the "non-magical/non-paladiny" cavelier/knight type.
--SD