Ballotpedia is already collecting and making available lists of 2018 candidates for all statewide office, in all states, even though filing deadlines are still in the future in almost all states. Ballotpedia searches for information about candidate announcements to help compile the list. Once the filing deadline passes in a particular state, the page will be amended to just show the candidates who actually got on the primary or general election ballot. See here. In order to obtain the list for any particular state, one must input a particular residence address for that state, because apparently the editors thought this tool is only interesting to people to learn about their own state.