Create new repo on Bitbucket from Git Bash terminal?

You can use the Bitbucket REST API and cURL. For example:

curl --user login:pass https://api.bitbucket.org/1.0/repositories/ \
--data name=REPO_NAME

to create new repository named REPO_NAME.

See Use the Bitbucket REST APIs for more information.

UPDATE

For Bitbucket V2 specifically, see POST a new repo