  Optimal rank-metric codes in Ferrers diagrams can be used to construct good
subspace codes. Such codes consist of matrices having zeros at certain fixed
positions. This paper generalizes the known constructions for Ferrers diagram
rank-metric (FDRM) codes. Via a criteria for linear maximum rank distance (MRD)
codes, an explicit construction for a class of systematic MRD codes is
presented, which is used to produce new optimal FDRM codes. By exploring
subcodes of Gabidulin codes, if each of the rightmost $\delta-1$ columns in
Ferrers diagram $\cal F$ has at least $n-r$ dots, where $r$ is taken in a
range, then the conditions that an FDRM code in $\cal F$ is optimal are
established. The known combining constructions for FDRM code are generalized by
introducing the concept of proper combinations of Ferrers diagrams.
