WhatsPlaying is running the rumor that Terminator 5 will likely be a reboot of sorts. Instead of continuing the events of Terminator Salvation which had been originally planned. Taking influence from successful franchise reboots like Star Trek. From what I gather from this could mean either a complete reboot or a continuation from the events of T2. Director Justin Lin has previously stated that he doesn't consider any of the non-James Cameron films canon. So we should try to forget everything that happened in T3, The Sarah Connor Chronicles and Salvation.
When it comes to casting Arnold Schwarzenegger is expected to appear but it's unknown the amount of screen time he'll get. Considering Arnold has put a pause on his film projects to focus on his personal problems. Vin Diesel has been mentioned for either the lead or a new Terminator. Now there is rumblings of Paul Walker being up for the role of Kyle Reese. While Diesel seems to be returning to his action roots, Walker on the other hand is just a terrible actor. Hopefully the names become a little more inspired than the ones currently linked.