If Deckard is a replicant, surely he'll be dead "some years after the first film concluded", so an Indy 4 scenario seems largely redundant.
Not necessarily. The 4 year life span was deliberately engineered in. Once the replicant was fully grown it couldn't be reversed, but there was no reason a replicant wouldn't live longer if it was grown that way from the start They weren't sure whether Rachael would live longer than that, and maybe Deckard was designed the same way.
All I'd do, either way, is drop down on my knees and plead with whoever does it to do it just the way they did the first one. Don't turn it into an action movie, and don't use CG except for the big city flybys. I can't (or rather, painfully, I can) imagine what Blade Runner would have been like as shot by Michael Bay, with some cheesy greenscreen behind every doorway and a plasticised CG background dropped in.
The other problem is, how do they reconcile all the clunky 1980s computer technologies with the fact that this movie would have to be set in the 2020s?