After watching a full game of Vanguard Zero, I've come to a few conclusions about what's going to be busted and what will be balanced.
First, I'll explain some core mechanics so you can follow what the game plays like.
You have a 40 card deck. A vanilla starter, 13 grade 1s, 13 grade 2s, and 13 grade 3s that are also trigger units (you get to choose what type of triggers they are). This next part is conjecture, but I can only assume that you are limited to 4 heals. If we get to use 9 critical triggers, that will be fancy. I may even evaluate if running fewer heals and more crits would be worthwhile. Spoilers, it probably will be if my theories are remotely accurate.
The board is setup the same as vanilla Vanguard. You have a Vanguard circle and 5 RG slots in the normal locations. Riding and calling function identically to vanilla Vanguard. That's about where the similarities end. When it comes time to attack, you'll notice that you are unable to call Guardians. That's because Guarding isn't "a thing" in this game. Instead, your opponent must attack "interceptors" before they can swing at your face. This means that front row units are going to be consistently played and removed throughout the course of the game. Being able to maintain a board is key, as it is the difference between getting donked in the face once per turn or several times in a turn. Naturally, retiring a grade 2 on your opponent's field during your main phase leads to you getting an extra swing at their face, so retirement is pretty good.
For actual combat, drive checks function normally (other than grade 3s having 5k shield value and being triggers now). Damage checks are wildly different, however. When you take a damage, your Vanguard gains power equal to the shield value of the damage checked unit (which is always 5000 currently). If your opponent cannot hit 16k, then 21k (assuming you have no front row grade 2 RGs and they don't hit triggers), then simply damage checking can shut down their offense. Additionally, perfect guards do exist in the game. They don't require a discard to use, and automatically nullify the first attack done at your Vanguard that turn while they are in your hand.
Speculation: It was not mentioned whether or not that attack needs to have enough power to "force" the activation of the null guard. Something to look out for in the future.
With those core mechanics out of the way, the rest of the game functions like Vanguard, and games are won by going to 6 damage. All of the units I've seen have had their original skills (barring Dragonic Overlord, which gets Drive -2 on VC), but that is likely to be subject to change after more testing is done and they've already made the claim that cards could change for balancing purposes.
With all of that out of the way, let's talk some OP (in theory) units from the older sets.
Dragonic Overlord (on RG) - Dragonic Overlord was always a strong RG in early Vanguard in my opinion. I doubt they'll allow deck mixing, but if they do, he had some nutso synergy with Mr. Invincible, Hungry Dumpty, and Claydoll. In Vanguard Zero, assuming you have an 11k RG row and a Waterfall on Vanguard, you could easily CB3, wipe out the interceptors, swing 11k to VG, swing Overlord to Vanguard, Swing Waterfall to Vanguard and get in three damage that your opponent can't really do anything about (unless they have a null guard or damage check a draw and draw a perfect guard). That's devastating.
Dragonic Kaiser Vermillion - Similar in concept, attacking the Vanguard and the Interceptors at the same time is pretty good. In OG Vanguard, you would lose 10k~ shield. In Vanguard Zero, you will lose your face to the two subsequent attacks that you can't guard as long as the RG rows can get the needed power to hit over the damage
checks, which is the core concern Narukami seems to have at the moment. Additionally, Shiden and Raiden "stopped interceptors" in OG Narukami. Interested in seeing how those transition.
Bonus Critical Vanguards/Rear Guards - It should be pretty obvious that bonus crit units are cancer when not only can the opponent not guard, but they also must use their perfect guards as soon as they are able to. If you aren't attacking Vanguard last, you are heavily incentivized to pass crits to your RGs (as the only thing that could make the next attack be nullified would be them hitting a draw trigger (10% if they run 4) into a null guard (10%) which comes out to a 1% chance for them to do so, so giving them less damage checks on the between is going to be extremely beneficial. If the unit in question can hit 3 critical, it's just game over at that point.
Look to Gancelot either being overpowered (trading a trigger back to deck for an interceptor is already strong, especially since you pick which units are which trigger and this will be your heal. If he can CB4 for +10k +2 crit like in OG, it's game over) or nerfbatted/changed completely.
On-hit units - This also shouldn't be much of a surprise to anyone. If you control when your opponent is able to guard, you control when your on-hit effects proc. The majority of on-hit effects in the early game were either CB2 draw a card effects or Megablasts (draw 5 cards/retire all opposing RGs), so this won't be an overwhelming advantage necessarily. But the alternative Enigman builds from set 8 would certainly be stepped up quite a bit (made worse with Laurel being a card) and Aqua Force would also get some solid usage out of Hydro Hurricane Dragon. Things to keep in mind.
At the end of the day, these theories are based largely off of what we've seen so far and what is likely to be coming for the game in the near-ish future, assuming major changes to cards aren't in the works (they probably are).
Some cards aren't even playable without grade 0s (RIP in Aleph). So there are definitely going to be changes. Hopefully Bushiroad doesn't screw up the game balance super early, but they're Bushiroad, so... they probably will.
