With Massachusetts’ top gaming regulator under investigation and Boston’s mayor suggesting that the awarding of the Boston-area casino license last year may have to be revisited, Mohegan Sun officials say the Mohegan Tribal Gaming Authority remains “open to opportunities.” The authority, which proposed a casino in Revere, Mass., lost out to Wynn Resorts’ plan for an Everett project in a two-way competition. Mohegan Sun joined the City of Revere in suing the Massachusetts Gaming Commission over its awarding Wynn the coveted license.  The City of Boston, contending it should have been regarded as a host community for the Everett project, also sued the commission.