You know what might be fun?
It isn't the most high-stakes DLC idea ever (I mean, it's hard to top the local version of "saving the world"), but recently reading a history of Boston's WBCN from the 60s through their recomissioning as an all-sports station gave me what I think might be a fun idea.
It's just Diamond City Radio, doing DCR things, but with loads of side missions:
-Piper tries her hand at moving from print to radio journalism.
This, amazingly gets her in even more trouble than usual, since Publick Occurences calls on people to be able to read, which some people can't do. Maybe it gets her back on the radar of the Children.
As the biggest news: Following the endgame, there'd surely be an election for a new Mayor that would involve all sorts of shady business to uncover (or, instead, get involved in. Or both.)
In the wake of recent events, maybe Hancock is given just enough room to move- probably not directly running, himself, but being a shrewd operator even when he can't just kill a guy.
-There's the issue of finding new advertisers, which could be a bunch of quid pro quo deals with vendors all over the place, or just a bunch of persuasion tasks. Maybe there'd, finally, really, at last be a sale on brooms!
-Travis is of course still the headline act, but this new fame leads to... stuff. Stalkers, blackmail, attempts at payola as weird as the time that guy tried to mug you in the wasteland, all that good stuff.
(Stealing from the WBCN example, there was an incident where a DJ was slipped some drugs in food given by a fan. He finished his shift, amazingly and kinda hilariously. The idea of an even-more-incoherent-than-the-first-time Travis hanging on until the Sole Survivor can find an antidote or something tickles me.)
-Sheng Kowalski gets a turn, with or without Travis dying.
-There's a bit of rivalry with whoever took over the GNR franchise in Goodneighbor from Kent. This involves a bit of looking for 'exclusive' talent a la Talent Pool , or searching for old holotape recordings to scrounge allowing more actually Massachusetts 50s era acts, which could be something awesome. (Boston has a really rich music history, and beyond the 50s thing, there's some neat stories you could crib and make adventures out of.)
-Depending on what faction(s) survived, a Brotherhood station or the Institute's station could be an interesting counterpoint as well.
-I can see some of the other characters doing 'bits': Codsworth with a 'thought for the day' cribbed from half-burnt books you recover, Moe Cronin doing the 'sports report' from last night ("Some terrific ultraviolence to talk about. Real first-round Hall of Fame stuff..."), Curie doing some basic health PSAs (maybe as part of an associated side-quest to help people in the area), a "Sunday Morning Religious Roundtable" between Pastor Clements and any number of other people (a Children of Atom rep would be a fun 'guest').
-Preston would probably not be a full appearance (he "works" for Radio Freedom, after all) but it might be funny to see him cut off while trying to tell people that when they're done with what they're doing....
And so on. Without getting too "WKRP" about it. Rather than being a high-stakes thing, it would seem like just a bunch of picaresque slice-of-life stuff that just added up to a lot of decent-sized 'results' (changes in the setting).
This leaves alone stuff like fixing sabotage, or trying to boost the signal a la the GNR things in Fallout 3.