I really hope it won't be T2 or E.T. because for me they are both classic Universal. I do think both could use some tweaking and an extensive refurb of some kind but both attractions are still incredibly popular. T2:3D has always been one of my favorite attractions at Universal and I still think it's extremely enjoyable, no matter how many times I see it.
I know it's unlikely but perhaps Universal could add more attractions before they start taking away the classics. There's quite a lot of unused space around the park that was once home to an attraction of some kind, so why not bring something new to those? Again, I know most of them are merely soundstages but it's space that could be used for good.