Given Disney's recent track record, i'm not inclined to give them any leeway with drastic changes to existing attractions anymore. Even your Small World suggestion. I gather you propose they move it to Epcot right? In a different era, I might agree. But if they ever closed Small World for real, it's far more likely we'd never see it again at WDW (or at least not anything like its original form).
TSI and the Riverboat are non-negotiable for me, regardless of which era of Disney was handling projects. They have AMPLE unused land to the north of Big Thunder, land that was once officially designated for expansion. They would again need to address the walkway situation and engineer a solution across the canal feeding into the RoA (would need to be a drawbridge so the Riverboat can get to the dry dock). But if they're going to expand that area of the park, that's what they should work towards. I'm completely against anything that would fundamentally alter the island/river and dock the riverboat.
Here would be a portion of my rough solution. Red being the general area of possible expansion and yellow being the walkway. Doesn't have to be exactly like this, I've seen similar ideas from more talented armchair imagineers such as idealbuildout, but that's the basic idea. And it would be far less destructive to the river or island.
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I had even more extensive ideas across the northern bank outside the train berm that involved diverting the canal further north and filling out some of the space above Haunted Mansion and Small World (though I don't think there's official expansion plot above them and it's nearby the fireworks launch area). But i'll keep this diagram simple to start with. We're also getting way off topic, there's another section for armchair imagineering.