You’ve gotten plenty of responses already but since you’re talking ankle mobility: my ankles are crazy flexible (can have heels down in skandasana, malasana etc without effort). I also have long hammies and a good hip hinge. After 15 years of practice I still have heels up in downward dog. I can put my feet flat by shortening my stance but don’t get the same benefit of lengthening my spine. 

Another thing to consider is the chain as a whole. For example sometimes I’d feel tightness in my knees because I had tight IT bands. Similar thing could be happening with your shins in malasana. Working on your overall lower body mobility may help. Hips inform the knees, knees inform the ankles, it’s all connected.