I'm not sure you're aware but playing players at positions that they have never trained for is synonymous with Pakistan, essentially the opposite of what you just said. A good example of this is debuting Fakhar as a slogger at 6 when he racked up scores in List A as a top order bat. Also, in terms of being dynamic, we even tried opening with Faheem of all people in NZ during that 2018 series. It's also not that Pakistan doesn't experiment with floating players around the batting order (Haider Ali a recent example, while Hafeez, Sarfraz, Umar Akmal in the past), it's just that those experiments rarely work out. We tried loads of things before Imam-Fakhar. Needless to say, none of them worked out.
I can see why you would want Rizwan to open the innings, but the question is why not address the question of the middle order first, and then see how you you could re-jig the opening combo. Let's say you drop Imam (which would be ridiculous) and open with Rizwan, who are you planning on playing at 5, who can consolidate well, rotate strike to inject momentum into an innings and negotiate collapses?
Imam is coming off a decent SA ODI series (2 50+ scores) and Fakhar too (2 100+), so why mess with something of lower priority, if at all (i.e. top order), when you have more impending problems to solve (middle order & lower-middle order). Learning to prioritize is quite essential.
