I had a few ideas that could be combined or done separately.
A roaming world boss with a ridiculous amount of health, something like 1,000,000+. This would be something for two things: playthroughs with lots of companions, sim settlements conqueror, and FCOM, or for people who simply want an enemy to hunt for a long time throughout their playthrough. The world boss would be done in "phases", and it would be sort of like a hunt for the player. A player could kill it in one go, but the boss retreats after a certain amount of damage has been done. Maybe it can spawn smaller, weaker versions of itself to keep the player busy while it runs away. Just some spitballs on how it could work.
Secondly, I would love to see a Grunt Companion. I assume this would be done with all new skeletons and almost custom everything, so this could be a new race added to the game, with perhaps one or more factions that are either friendly or hostile towards the player, but the primary desire is the addition of a grunt companion that has an affinity quest centered around finding food nipples around the commonwealth. The grunt would enjoy things like the player finding comic books, listening to the radio, and collecting figures and bobble heads, since grunts in lore enjoy and consume human culture. After certain affinity checks, the grunt can mark the location of another food nipple. Hell, I'm not opposed to having friendly NPCs use the same dialogue they do with dogmeat for the grunt. Cait calling your grunt companion a "dirty little mole rat" would be way funnier than her calling Dogmeat that. Also, I am offering to voice such a companion, if anyone at all is interested in doing this.
Lastly, and this can tie into the companion mod, but it does not have to, adding a Grunt Goblin from Halo 5 or Halo Wars 2 to Fallout 4 would be amazing. The grunt goblin could be a ridable mech for a grunt race. It could be the reward for completing the affinity quest associated with the companion. It could be something as "simple" as splitting it in to parts and having it be a custom automatron the player could build, making it unaffiliated with any race or faction. Though, it would still need a custom voice added if it were to be an automatron. The automatron option seems easiest and most likely of someone picking up and doing, because mods already exist that add in the mantis, or even hunter parts as buildable on automatron robots. I think itd be pretty awesome to have a Grunt Goblin assigned to a settlement, having it roam around protecting the settlers.
If anyone is interested or has any questions or contributions, let me know! I have midterms, and I am full time, so I cannot offer anything more than opinions, clarifications, or help with voicing any characters, but I will respond to as many people as I can that may be interested. Again, maybe these ideas can merge somehow. I explained how the last two could, but maybe the first and final may merge as well. Perhaps YapYap the Destroyer is roaming the commonwealth in a giant grunt goblin, and each time he flees in cowardice, he sends out 3 normal sized grunt goblins to fight you in his place.
I am willing to help any way I am able to. let me know what you all think!