....potentially return in future instalments of the franchise.

Think about it, the voiced protagonist, the lack of (as many) RPG elements, even the name of the Sole Survivor hints at them being the only Fallout protagonist to survive forever, they chronologically survive all of the games. They survive the Great War unscathed, all the events of Fallout 4 and it's DLCs. The Lone Wanderer and the Courier, of course it is up to the player, but chances are canonically they are dead, be it from the intense levels of radiation poisoning they survive or the violent combat situations they find themselves in, even dying when they become elderly. The Sole Survivor is the only protagonist that really does have something to live for after the events of the game. The Lone Wanderer already toppled the Enclave, and the Courier, even in the Independent ending, has no real use because Yes Man truly runs New Vegas after that game. The Sole Survivor runs the Minutemen/Institute and is the Leader of both factions.

Combine that with the fact that the Sole Survivor can free Lorenzo Cabot to get a lifetime supply of Immortality Serum, and it's debatable that the Sole Survivor has a higher chance of returning than all the other protagonists. The Vault Dweller was mentioned in Fallout 2, and the Chosen One was a descendant of him, so possibly the Sole Survivor can be mentioned in the next game or even as an NPC?

What do you guys think?

Interesting Idea. I've always thought about the sole survivor being quite literally named because he supposedly survived the great war.

But what you say does hold some truth - especially since you're right about Cabot's Serum. I don't recall if there were other ways to immortality for any other vault dweller / courier.

I think we might see him either as a NPC in a side-quest, or maybe as minutemen general in another game.

The only problem is: how far could the sole survivor travel? Or how close to the commonwealth would a future game have to take place to warrant either a short cameo or a major role in the story?

As you said the role-playing options where quite limited, so I could imagine a dialogue that determines how we would meet the sole survivor in a future game. Something like "Have you heard of the Sole Survivor, I here he's destroyed the institute?" And your dialogue choices reflect which choices you made in F4.
All of which would change how you'd meed the sole survivor.

The problem with this would be how to integrate those choices into a game. For example if you've destroyed the institute (and presumably the new game takes place on the east coast) there'd be no enemy synths. Or if you destroyed the brotherhood then they'd be gone. Or maybe different settlements would be in the hands of different factions at the beginning of the game.

A lot of potential, but also a lot of work involved. I doubt that bethesda would go for it.

... speak with his own voice in this franchise? The first thing that came to my mind when I read the title  

Hm, this is one theory I've never thought about before. It could be possible for Bethesda to continue with these same voiced protagonists since all the hard work that went into Fallout 4, but I'm not too sure. Don't get me wrong, I may not have liked the voiced protagonist idea but the voice actors did a phenomenal job, much better than I had original thought. I just think that Fallout, like many RPGs, work a lot better when they have a clean slate. With a new fresh character, it's easier for a gamer to immerse himself/ herself into the world. I won't say it's not impossible for a sequel of Fallout to be a continuation of an already established character, but it would atleast be interesting.

@"Corvo" That is a very interesting theory. The Sole Survivor gets lots of options to stick with a faction in FO4. However I only think he would fit into another installment of Fallout is if he was on his own living in a shack or settlement without having any allegiance to a certain faction. Personally in my opinion, if they restricted the sole survivor to one faction it would completely ruin the purpose of roleplay and considering how Fallout 4 had quite a lack of Role-play. They may just add him in as a simple lone wasteland survivor wandering the wastes or a simple settler in some kind of settlement.

this is a cool theory i hope your right ...but i still hope they never do a voiced mc again

I very much like this theory, there's also nothing stopping the institute from taking over vault 111 and using the cryo tech, who knows what could happen with what's been added in fallout 4 with the serum and the cryo-pods. but what about the courier, he/she does potentially end up in control of Big Mt. watching over it, who knows what could be invented. i think there's a lot of possibilites for the courier to lengthen their life, as well as the sole survivor

there's also the possibility of the sole survivor being a synth i suppose