I understand that, without the parentheses, the correct form would be "one or more things". However, if one wants to make "(or many!)" a parenthetical remark, should that affect the agreement of the noun? The latter form "one (or many!) things" seems correct when read out loud, but the structure "one (...) things" seems strange to the eye.
Alternatively, would "one thing (or many!)" be an appropriate way to escape from this situation?