Personally for me should be like this for pro versions
256gb base $999
512gb extreme storage model $1199
1tb extreme hardcore model $1399
Would be nice but Apple would lose serious profit from upselling. But i think we'll see future 1 TB iPhones in the next 1 - 3 years. Usually they offer the next higher storage capacity on iPads first.
For example, you could get the iPad with 128 when the iPhone had 64 as the maximum. (iPhone 5 / iPad 4) When you had 128 GB on iPhone, iPad had 256 GB. (iPhone 6s / 1st gen iPad Pro) When the iPhone had 256, iPad had 512. (iPhone 7, iPad Pro 2nd gen). Now we have 512 GB on iPhone (iPhone 11 Pro) and the iPad already has 1 TB. (iPad Pro 3rd gen)
It's coming... and it will be very very expensive.