Smart Fill

Smart Fill will fill and attempt to retopologize a hole in your retopo mesh. The more evenly spaced the surrounding faces, the better the result will be. You can also use Erase and Insert Edges to reconfigure the result if necessary, as it may be quicker to repair the result rather than manually subdivide the entire hole after a Fill Face. To use Smart Fill, select at least one of the

Quad Fill

Creates a set of mostly quad faces to fill an opening. Only one of the open edges need be selected to fill the hole.  

Available from Edge or Face mode.

The row of images below show a Quad Fill to close an opening and then a Curve command to curve the faces created by the Quad Fill.

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