Don't know if you are still checking your post, since it has been awhile, but there are some great options even while you are pregnant.
First, I would highly recommend a ring sling. A ring sling, being adjustable, will expanded with your pregnancy weight gain and contract with your post-partum weight loss. A ring sling is easy to pull "open" which makes it easier to put a toddler in and not get in the way of your baby belly. Also, your toddler could ride on your hip or back and not be in the way of your belly. Once baby arrives the ring sling would work for your toddler and the newborn.
While a ring sling is terrific for short carries (and so much more comfortable than trying to use your arms) it can get fatiguing because most of the weight is distributed only on one side of the body and one shoulder. For longer carries (over an hour) a mei tai or wrap would be more comfortable. During pregnancy you can tie under or over your belly however, given your build, I'm guessing that wouldn't be comfortable for you - maybe? However, once baby is born you could definitely use a mei tai or wrap for both your toddler and your newborn. And if you can get one of your children on your back it would leave your arms and hands free for the second child.
(Oh, and I don't think a structured carrier like an Ergo or Patapum would fit you either.)
M'Liss