Maximum storage on iPhones have consistently doubled every 2 to 3 years. That there would be an 1TB option is not at all a surprising thing.
12 Pro / 11 Pro / XS: 512GB
8 & X / 7: 256GB
6S / 6: 128GB
5S / 5 / 4S: 64GB
4 / 3GS: 32GB
3G / 1: 16GB
Notice the 2-2-3 pattern. Given that a bigger storage option is an easy money grab for higher-end units as far as Apple is concerned, it would be highly unlikely that the the Pro models of this year's iPhone won't come with double the storage.
That being said, I am eagerly waiting for this upgrade solely on the storage options alone. I realize I'm in an extreme minority, but I do not trust iCloud for storing photos. Thus I have more than 138,000 photos and 2,100 videos shot with the phone on my 11 Pro taking up 448GB and leaving only 22GB of space left. I've been able to leave the photos on the Camera Roll since the 6S days thanks to the storage expansion keeping up with my growing collection.