Oh look, I go to sleep and something interesting happens
I don't seem to be getting anywhere by repeatedly going against the trend, but I absolutely want to make this clear: I don't think that we are going to get anywhere, or fulfil the concept, purely by trying to make the typing compensated for by its usefulness, or otherwise, in an area outside its own. Let's say we have any Pokemon with a type combination that makes it offensively incompetent or leaves a little to be desired, but has a heavy offensive build, so we can't immediately fall back on the classic "bulky thing so it's good" approach. Something like, say, Durant. Bug/Steel is a poor offensive combination, but it has high Attack, Speed, and Hustle. So, it's a pretty decent offensive thingamajig. If it had a wee bit higher stats and Earthquake, then yes, it might just be OU. But that isn't really saying anything. Similarly, we ourselves could create our very own Poison/Steel rampaging behemoth, but... would it really be worth it? What have we done other than make another powerful offensive Pokemon? What have we learned?
We've already had this story with Necturna. While it was undoubtedly a nice learning experience, what it came down to in the end was a balancing exercise with a pre-determined condition. What I wanted to see here was a typing that was indeed "a mixed bag" - there is no such thing as a typing that is inherently bad in all respects - such that it was usable for its positives regardless of its negatives. In the options we have at present, we appear to be making a typing whose sole benefit to its use comes from being good generally but not for the purposes of the CAP itself. So two paths result. Either the movepool, ability and stats pick up the slack entirely, or we make another generic bulky attacker. Again.
Personally I don't see what there is to be said that hasn't been said already. We appear to have three choices before us; each of them focuses the concept into something that will allow for us to all be on the same page with regard for what we are trying to accomplish, though at the same time limiting us. I don't see what else productive can come of this thread.