There is no reason that future TMP stories would have needed to be identical in tone to TMP, but I agree that early TNG gives us an idea of what we would have been looking at - a good foundation for what was to come but largely mediocre, although a higher movie budget and Devil's Due could have been an interesting romp.
I would have liked to see more of Decker and Ilia actually, if their encounter with V'Ger had been hard-baked into their characters instead of using a reset button, similar to Picard's encounter with the Borg. It could have added a really interesting dimension to them - are they just modified probes transmitting data back to V'Ger? Maybe I'm just projecting Boomer from NuBSG...
I would have liked to see more of Decker and Ilia actually, if their encounter with V'Ger had been hard-baked into their characters instead of using a reset button, similar to Picard's encounter with the Borg. It could have added a really interesting dimension to them - are they just modified probes transmitting data back to V'Ger? Maybe I'm just projecting Boomer from NuBSG...