Serious Reply Warning!!!
The Spear of Destiny would be a horrible choice in my opinion. They've already done The Ark & The Grail, it would just come across as a desperate, and week attempt to reclaim the 'glory days'. In fact, I think I read somewhere that they scratched that off of the pad pretty quick with the last film.
I think the Jones family should go after retirement next. No, I'm not being a wise guy. I honestly think that there is nowhere to go but down with this franchise, and the last film did a great deal to prove that.