Hrm.....Well, I guess I misinterpreted what y'all were looking for in "undervalued"...
So, let's try this: Grass/Ground. We only have one example of a Grass/Ground type, Torterra, and while it's certainly an ok Pokemon, I don't think anyone will disagree that it's not exactly a top OU threat. Grass/Ground has more weaknesses than resistances, being weak to Fire, Ice(X4), Flying and Bug, all of which are fairly common attacking types, particularly Fire and Ice. However, It's two resistances, to Ground and Rock, are interesting, as it mitigates the damage from Stealth rock, which is conducive to our utility counter concept, and gives it a respectable number of opportunities to switch in on things. Most teams carry at least a handful of ground and/or rock type attacks, and usually use them to nail specific threats on the enemies team; having a mon that resists both can nicely turn the tables on anyone trying to take advantage of the wide coverage Ground and Rock offer. And, we can't forget it's immunity to Electric types, which saves us from Thunder Wave and gives us another good opportunity to switch in. Offensively, Grass/Ground is a bit of an odd choice, as it's actually resisted by a pretty good number of Pokemon. However, most of these aren't competitively viable, or are rarely (or never) seen in OU. While it's true that most of the Pokemon that resist Grass/Ground could threaten our CAP in turn, I actually think this helps go toward the "bad, but not unsalvageable" premise. This thing has flaws. Pretty big flaws. But at the same time, it has ample opportunity to shine if it's used right. To me, it seems to hit everything we want: A poor but usable typing, while still maintaining potential as a utility counter.
As for other types that have been suggested: I love the idea of Bug/Dragon, but just going off what Imanalt, HealnDeal and others have said, I'm a bit uncertain about anything/Dragon. I mean, for a "bad but not unsalvageable" type, it's got an awful lot of useful resistances, and while it does, as Sparktrain mentioned, have a hefty number of weaknesses, it is not resisted by anything it's weak to, except Fairy types. Only 61 Pokemon resist Dragon/Bug, it has massive neutral coverage, and it can maim just about any Grass, Psychic, Dark, or Dragon type in meets. In my opinion, Bug/Dragon is a pretty decent, if not just plain good, type. Honestly, I would love to see a Bug/Dragon type, and I think it definitely deserves to be explored with a CAP...just not this CAP.
I'm actually a bit perplexed by Electric/Ghost. Sure, it's weak to Ground and almost certainly won't get Levitate. Sure, it's vulnerable to Dark type moves like Knock-Off, as well as Ghost Type moves. But, it also has 5 really useful resistances AND 2 immunities. Even if one of those immunities is to Normal type, it still has a grand total of 7 resistances and immunities, something just about any Pokemon would kill for. Additionally, Electric/Ghost has just ludicrous coverage: only 13 Pokemon resist it, only six if we just count fully evolved mons, and only two of those (Hydreigon and Krookodile) can really be considered threatening/ Give it Ice Beam, or hell, HP Ice, and it's good to go. I think what you said about Rotom is very telling: It' NU, but it has poor stats. I'd be willing to say that if you gave Rotom a better stat spread, you would find that Ghost/Electric is actually a very good type combo. Of course, I suppose the same could be said of any type combo, but that's neither here nor there.
I know i was against it earlier, but I'm actually starting to lean toward liking Poison/Rock. While it does have six resistances, two of those being to Normal and Poison, which are rarely seen offensively, kinda reduces it's defenses from "great" to just "good". Being 4X weak to Ground really hurts it, and weaknesses to Water, Psychic and, to a lesser extent, Steel makes it hard to switch in whenever you like. You have tough enough defenses that you can feel confidant switching in, but you can't just switch into anything, either. Offensively, Poison/Rock is good, but not great. There are an awful lot of things that resist it, many of which are rather common, so I don't think we have to worry about being to threatening. We can switch in on a lot of things and threaten them, but there are still plenty of things out there that our CAP wouldn't be able to handle without some coverage, which I think lends it well to the "Bad, but not unsalvageable" idea. I kinda like Fire/Rock for mostly the same reasons.
That's my two cents.