i am sure i played in at least one tournament where this was a thing, and i have to say it made playing so much better in my opinion. i remember being rather sceptical about teampreview when it was first introduced, because as we know, new things are always bad things, but as time flew by, i learned to appreciate knowing what the teams look like and not having to guess while planning my game. with that in mind i think dpp would see some good changes:
first of all, the leadspot loses so much importance. machamp for example is one of the best leads in my opinion; i value it because it almost always reveals at least 2, if not more mons of the opposing team, information i can work with and base my plays on. if this trait becomes worthless machamp might not see as much usage. same with azelf and it's rocksset, since you can build in countermeasures for that more easily now.
but it's not just the viability of single mons going down, it's rather the whole "turn 1 match up" situation that changes so much. just think of using ttar as a lead. sure it gives you a very good situation to move on with the game, by getting up rocks and forcing a few mons out, even killing stuff, depending on what the set you use is, but that only works if the opponent uses the leads you hope to see. once the opponent leads with machamp for example, you know you'll be trying to catch up from turn 1. while good teambuilding in a "no preview metagame" limits the caused damage, it's still nothing desirable. team preview obviously changes that, since you can basically counteract to whatever you expect the opponent will lead with out of his 6 mons. no pressure to think through million lead match ups while teambuilding sure is a good thing, cause it loosens some restrictions while teambuilding.
i don't think overall teambuilding would change too much (outside of the above mentioned lead section), because balanced teams will still work the same way (and be even better), offense will still use the same combinations to break through teams, though a few mons might become worse, cause scarfers won't give set up as easily as they do (i mainly refer to mons like lucario). i think stall would get way better, cause no more random magnezones popping out of nowhere, since you can now at least attempt to double out or something. also longterm planning is easier with preview.
very often have i wished that dpp was already with preview, i think it would make playing a lot easier and less random, because fuck having to guess.
edit: i probably exaggerated with my choice of words, but i didn't mean to imply dpp is random wild guessing