Oh, please, no.That's what they did with TOD and it did not work. If anything, they need to lighten things up.
I wouldn't say "darker", but I would like to see it become a bit more serious and grounded like the first movie, where the quest had a real sense of importance about it and villains felt like a real threat and were not played for laughs.
I certainly wouldn't object to another TLC if they could actually make it as fun and clever as that movie, but after three movies of that lighter tone I think I'm kinda ready to see the more serious Indy again.