congrats on the loss.
Hit some big exercises (squats, deads and the sorts). That will help get the core nice and strong. Build you that big solid muscular core which will fill in some of that loose skin.
You can’t spot reduce-doing sit ups till you are blue in the face won’t reduce abdominal fat. I couldn’t figure out if that is what you were asking, or if it was that you wanted to increase the size of the abdominals muscle to fill in the loose area. So no offense if i’m telling you stuff you already know.
Lastly, after a big weight loss, sometimes its better to stay at maintenance/ever so slightly above maintenance and lift hard for a bit before going after that last little paunch. Alot of people get overzealous after losing a ton of weight to get that 6pack and get drastic and end up losing muscle and still no 6 pack. I think it was shugs who talked about set points in one of his skinny fat stories (or I could be making a connection thats not there).
But anyways-in a simplified and easy to think about way-your body likes equilibrium- if your 250lbs your body likes it there, so when you lose weight your body starts to worry and go back to its nice equilibrium it was set at. But if you maintain at a new weight for a while your body starts to recognize that as the setpoint and then you hit it with another cut.
Just my 2 cents. Whatever you decide tho, be proud of what you accomplished so far- 70lbs is a nice feat.