Didn't say my opinions on the subject as I had to be quickly be someplace. But I'm back so:
Damage & PP Management: This is an interesting element to have. My first thought is, are we still following the normal Raid rules when is comes to a Pokemon being knocked out? It's down for one turn and then comes back with full HP and all PP restored? If so then we come to an interesting situation here where, if you're up against the 3rd Opponent and your Pokemon is low on HP/PP, if your team is looking good on the win condition (not having spent 10 turns or gotten 4 KOs) then it may be worth letting your Pokemon be knocked out (possibly even by one of your own allies) so that it can come back fresh just before having to go up against the Legendary. Though the only other question after that is, if your Pokemon is knocked out on the turn that your allies finish the Raid, during the next Raid does your Pokemon come back with full HP & PP on the first turn or do you have to wait the turn you were supposed to last Raid? I'm going to assume the former, would be silly to force you to endure a punishment that was meant for the last Raid aside from HP & PP lost, but this is already a strange situation so you never know what GF would consider proper to include.
Helpful Characters & Berries: Now it only mentions these characters & berries are for "healing" purposes but that's both vague and could just be generalizing their role.
Focusing on the Berries first, is it just a simple case of "hey, you found a berry your Pokemon eats and heals up" (and if so, by how much? Does it depend on the Berries you find? Can it heal status ailments & restore PP)? Are there different kinds of Berries we can find (and if they are and we find a whole batch, do we get to choose which Berry we want to have)? If we want, could we possible equip the Berries as Held Items if our Pokemon isn't holding an item already (and if they are can we switch held items, a thought that has just crossed my mind I'll go into more detail later).
As for Characters, well that right there has a whole bunch of possibilities. Sure, healing is the most basic thing they can do, but a human character could provide other services: Offering to switch to another rental Pokemon they have, offering different Held Items, offering to teach the rental Pokemon certain moves, etc..
Also, assuming we encounter these Characters/Berries between the Raids, do we see on the path that "oh, this path has a batch of Berries/character" or is it random chance we go down a path and we RNG into them? We can already see which Pokemon we'll face down what path we chose, but if we can also see these helpful features it would add another layer of strategy where normally one path would lead to a Pokemon you wouldn't think about taking on how there's a NPC along the path that could provide you a major boon that would be helpful against the Legendary at the end.
Three Normal Raids Before The Boss: As
R_N pointed out, seems strange to cut it at three when there's four players and all may want a better Pokemon before facing the Legendary. Then again, if the idea here is to have players cooperating, maybe this was intentional. If it was four or more battles, then it means everyone would be getting a new rental Pokemon so just click "yes" when it's your turn. However, now we're told it's only three. Someone is going to have to keep the starter rental Pokemon, so after a Raid and someone is given the choice they actually think "out of the four of our rentals, is mine good enough to keep for the Legendary so pass this Pokemon to one of my allies or do I need a better rental so take this one". It now forces cooperation in every Dynamax Adventure, everyone has to think what's better for the group's chances against the Legendary.
One Legendary Per Cart: Nice to know it's a guaranteed capture once we defeat the Legendary, that relieves a lot of worries of at least getting a prize if you win in the end... if you haven't already caught it. One of each Legendary is interesting as that means the game is keeping track of which Legendaries we caught. So I'm also going to guess that means if we go up against a Pokemon we already caught then the only reward in the end is all the items we get (though would be nice for those players who already caught the Legendary for, just that/those players, the Legendary vanishes but in its place another Pokemon appears to let us catch (maybe even give it an increased chance to be Shiny), but I know that's a stretch). But hey, for me, now that I know there is an end goal in sight has renewed my interest in the campaign (at least until I capture all the Legendaries).
BUT this does also raise a question I don't thin anyone has asked: how exactly are these Raids going to be set up? Do we just go up to Peony, say we want to do a Dynamax Adventure, and he throws us into a random one (and we just have to hope the game would try to pair us with a team going after a Legendary we haven't caught yet)? Or will there be a kind of lobby where we'd get to pick a Legendary we want to go after (maybe each Dynamax Adventure they'll give us a selection of several Legendaries available and hopefully one of them would be one we don't already have, otherwise we'd just have to do a filler adventure to refresh the list or wait till the next day) or can join another player's party if they're going after a Legendary we want (or just to help out someone if going up against a strong Legendary; also wouldn't mind a "random" button if we just want to help out but don't care who we end up with). My personal hope is for the lobby, would make the most sense and be less frustrating.
And that's it for my thoughts on this reveal, except for this one thing concerning held items that crossed my mind:
Time Between The Raids: So, HP & PP loss is carried over between battles. Alright, but what about between battles? There's going to be 3 moments between each battle where we're travelling down a path, there's no time to maybe do another action or two before we start the next battle? Like I'd imagine if we're going down into a Raid Den we'd also make sure to bring a bunch of Potions, Medicine, Ethers, and/or Berries with us; maybe even some additional supplies like different Held Items so we could switch them between battles or maybe some TMs/TRs we could teach our Pokemon for the upcoming battle. I imagine if we have a second to pick some random Berries we come across or chat with an NPC who just so happens to also be down there we should have time to use some supplies to maintain our rental Pokemon.
Because this isn't a Battle Facility, there's no rules of conduct, you're going into a Wild Pokemon's nest and capturing them; at that point it's about survival. This is why I think this whole rental idea is stupid unless there's a good reason for it.
I don't want to bring up fears of Shiny Pokemon you can't get again, (that hurt my soul...) but I'm worried about if the Legendary is the same one every time you try that day like normal dens, (AKA, Same Nature, IVs, and if Shiny) or if you can see a Shiny and not be able to catch it because your partners have terrible Pokemon for it. (and/or you're spamming Life Dew with the Friend Guard Clefairy because you're that one idiot that doesn't attack ever... I'm so screwed... :P)
I'm going to guess you'll probably be given the choice not to capture the Legendary if it's not a Shiny. Infact, that is a bit of a concern, this feature would make hunting for a Shiny Pokemon extremely difficult (say what you want about the Ultra Wormhole Ride, at least once you found the Legendary you could save your game before the encounter). It would make no sense to Shiny Lock these Legendaries, so if I were GF maybe I'd put in a mechanic where, if you choose to not capture a Legendary you could have captured, the next time you encounter a Legendary you can capture it'll increase the chance of it being Shiny (though maybe just for you, everyone else could probably just be facing a normal version of the Legendary). That's really the only work around I think they can do for this, and it would have to be something they'd consider important which we know is likely not going to be the case. Sorry to say, but looks like USUM may still be the better game to Shiny Legendary hunt (even if Ultra Wormhole Ride has way too many mechanics thrown in for no good reason...).