Ab Exercises

Here's one that might work. Join a martial arts school, one with lots of kicks and active sparring and partner drills. I would recommend Muay Thai or kickboxing, it's trending these days, lots of gyms opening so you should be able to find one, but there are many options.

So here's why martial arts might be just what you need:
Getting your legs off the ground and throwing hundreds of kicks/week will do wonders for your core, so there's your muscle build-up, all the way around your core, without a single plank.
And a class of Muay Thai/kickboxing/western boxing/MMA can roast a thousand calories, so there's your fat loss. MT and boxing bring a traditional training method that's based on round after round after round of drills and bag work. You will work your tail off. Nothing works better than a sadistic coach. Add some fork control and you'll be there in short order.

Join that gym, and six months from now you'll replace that bogus default avitar with a shot of your six-pack. Being a lot tougher is just a bonus.

Other disciplines will do it as well, like boxing (no kicks but relentless conditioning), Taekwondo I hear is very kick-centric. There appears to be a marked difference between gyms that teach combat sports of today (including MMA gyms) vs martial arts of the McDojo variety. You can watch a class at any of them and you decide if everyone walks out completely gassed, or all pretty in their gi.

Good luck!

And for fun and motivation, no offense to the offendable: