Cameron&Schwarzenegger&Hamilton&Biehn or bust...
This is my take on the fundamental reason why I don't think Genisys will work, and what I think it would take to make another movie in the terminator canon worthy of the first two movies.
I love Arnold as the terminator in the first two movies, and even seeing him in T3 and TS was cool at times despite my strong dislike of those movies, but the termintor story is just as much the Connor/Reese family story as it is about skynet embodied in the terminator.
So it bothers me that Arnold will be on screen in Genisys with recasts of Kyle and Sarah. That doesn't sit right with me at all. To me it can only therefore be 50% of a termintor movie for me at best, and in a wishy-washy sort of way because if Michael Biehn and/or Linda Hamilton aren't there, as far as I'm concerned neither should Arnold be there. If they expanded the mythology by showing other types of termintors that don't look like Arnold that were sent for other purposes besides going after the Connors, and featured other characters, that I'd be more interested in than yet another T3 situation (Arnold but no other pivotal actor returning from the original).
I'd be supremely more interested in this movie if they cast Michael Biehn and Linda Hamilton. It's a team thing; terminator/Connors, not just in the characters but in the casting as well. I'm pretty sure Genisys is going to come off like another T3 at best, maybe a little better.
If they really want to get it right, there has to be a re-teaming of Michael Biehn, Linda Hamilton, and Arnold with Cameron producing; they are The Beatles of terminator movies. The reason I say Cameron producing is because I think he's lost his touch with working within confined budgets and doesn't have the drive anymore to make the story tantamount in his work, but he is still essential lifeblood to making a great terminator movie, and as a producer with the right director working with Cameron, I still think a true T3 could be great under these conditions even without Cameron in the director's chair.
An idea for how to approach this would be to see Hamilton as old Sarah now in hiding and living under a new identity, with Judgement Day averted, but the development of Skynet was delayed (the T3 idea), and John Connor is also in hiding and living under a new identity, and together Sarah and John are keeping an eye out on the development of a.i. John and Sarah emerge with a small group of people they've come to trust over the years as a new company begins moving forward with the development of weaponized a.i. John and Sarah and their small team fail to stop the new a.i. and civilization is again destroyed but John and Sarah and their team survive and lead the future war. They figure out a way to destroy Skynet before Skynet can make the time travel technology to send terminators back to stop the Connors, but John dies in the process. What's left of humanity (not much) must move forward to rebuild itself. Just an idea. I'm not saying it doesn't have problems or that it's amazing or anything. I'm not sure how you'd work in Michael Biehn, but maybe he could be in some flashback scenes from the original future war, and/or doing some narration. For Arnold, you'd only see young T1 Arnold as infiltrators in the future war, and the resistence reprogramming some of them to fight the bad termintors. And of course R rated.