Like what you do in Fallout 4 changes a few dialogue lines in Fallout 5, or characters make a cameo, something along the lines. Or else simply go with a default canon.

Way too far ahead to even begin thinking about that. Plus, no choices you made in previous games where there was an alternative was ever mentioned in the subsequent game, just the overall story decisions that played out regardless of the player's intention were mentioned.

Nope.

ME tried and failed.

The amount of writing it would require is staggering if it was to be anything but an easteregg.

There's a good chance Fallout 5 will not take place anywhere near Boston, so I would think that would be relatively pointless feature other than an NPC going "hey did you hear about the thing?"

No I don't think it'll happen. The wasteland is so spread out and isolated if Fallout 5 comes out and exists in another location its very unlikely our choices in Fallout 4 will matter at all. Also, we all know bethesda is terrible at making your decisions matter. You provided clean water to all the inhabitants of the wasteland? Well no one recognizes this and the water is still plenty irradiated. Also all the characters that knew about it with the exception of a couple BoS guys ended up dead. Skyrim? You saved everyone and we got one or two generic dialogues that may come up when you walk past

someone.

Another reason is bethesda is big on mods and a modded game of fallout 4 will not transfer to fallout 5.

Mass Effect worked because the game is linear, every character might care about what you did in the past but there weren't nearly as many characters as bethesda games.

Bethesda games aren't really the right platform for continuing a story across multiple games. They've never done it before and they probably never will.

Its not too early to discuss this, we can't discuss this after Fallout 4 is already out because by then it will be too late. This kind of thing takes a game in advance to plan out. But theres just a million reasons why this won't happen and no reasons why it should, so I wouldn't expect it.