MOG Dress

I'm shopping for a mother-of-the-groom dress for late next summer. I'm looking for something simple, perhaps with a 3/4 sleeve jacket. I'm a bit busty, plus size, with a waistline. What type neckline should I look for? Also, should I go for a regular waistline or higher?

I agree that a jacket is the best choice for you. However, don't look for something too simple. Remember that breaking up any expanse of size, with fabric, color, or detail, is a great way to provide narrower illusion. I often recommend a column of matching color (a top and skirt) and a jacket over, perhaps in another coordinating color. This would create 3 color blocks on your torso: the central one from the top, and the two sides from the jacket.

Your 3/4 sleeve idea is a winner, because it shows the narrow forearms, highlights bracelets, and still covers any upper arm issues.
If you look for a high waistline, remember to avoid one with fabric gathers below, as this often has an unfortunate effect. And if you have a defined waist, in a flattering proportion, there's no reason to mask it.

This dress isn't simple, but it has many flattering elements: the illusion of torso created by fabric, the portrait collar, and the straight lines of the skirt below.
The Mother of all MO dresses

Congratulations on your upcoming celebration!