there's an interview in the Megaman Zero Complete works that reveals that the developers believed that the Legend's series was the ultimate end point of the Megaman universe thus far.  Thus they designed Megaman Zero (and ZX after it) to hint at that. (in reference to you edit about the Vent-trigger similarity)

Also, Reploids are immortal, human's can be transfered into robot bodies, and Vent can certainly be Tigger!...although I suppose even Weil could be trigger too.  Technically.

I don't really believe that Vent is Trigger, but I do believe the developers were changing their Zero Art style to conform with the Dash art style, at least on a subconcious level.

Even if Vent was Trigger (which he isn't) you can't expect that someone who has lived a few thousand years to be that joyful.

Heh, this reminds me how at first some people thought Giro would turn into "The Master" from Legends 2.

arg.  I can't believe I'm about to argue about but this stuff, but who am I kidding:  I love conflict.

A 2000+ year old machine could, in fact, be joyous and naive IF he found a way--somehow-- to store all his past memory and emotional baggage in a little, itty bitty toy monkey...If only.

Reploids do not naturally age, but aging is induced:  Reploids were required to visit the Sage Trinity (specifically Albert) in order to have their life setting adjusted.  I don't think Reploids actually age, but have to undergo maintenance to simulate the aging process.  Consider Prometheus and Pandora:  Albert set their life cycles to about two days or so, yet we couldn't actually SEE them age during this period of time, and why would we? Robots don't age.

SPECULATION:

Now that Albert's gone, and we've seen Thomas  talk about resetting the world and bringing back the original bodies, perhaps the Sage Trinity will vanish, and the world will once again go back to it's war-torn self.  Which hero will rise from the ashes to lead this new world?  Vent's not a bad choice:  He no longer resembles X nor Zero, but his own, war ravaged character.  In fact, we see two Vents, just like we know that there are two Triggers:  naive Vent, who looks like Volnutt, and experienced Vent, who doesn't.  Just you wait, ZX3 will definitely include a little brown monkey!

If new type reploids do age, then Prometheus and Pandora might just be different types of reploids.  People often speculate that they're trapped in their battle modes, but I've never been convinced of this.  In fact, I don't think they megamerge at all--They just have old-style battle bodies, like the original X and Zero.  There's a cut scene that suggests a connection between the siblings and two model Ws, but I think this is an image of the siblings being brainwashed and manipulated, rather than them harnessing Model W's power.  Besides, the new reploids were made to be weak.  Why would Albert create his warriors from weak stock?