Plunge dps started with Xiao. In the early days, plunging was restricted to a few characters, dragonstrike Diluc, then Kazuha. But now 4.4 has brought plunging to the masses.
Similar to how hyperbloom gives a rookie friendly way to clear abyss (easy access to high dps), so does plunging dps with Bennett C6, XY, Furina. Now even Yun Jin can be a plunge master.
Note not every character may benefit from plunging (e.g. bows, some catalysts), so we generally focus on melee dps (sword, claymore, polearm). And some characters with infusions that cannot be overriden (like Xiaogan geo infusion after E) may not be able to infuse pyro with Bennett C6. Moreover, claymores (which have larger hitboxes) may be able to benefit more from collision damage. Certain characters with higher plunge multipliers (Diluc, Xiao) may capitalize more from plunge dps than others. Destruction characters that can manipulate hp may also synergize better with Furina. And the character should synergize with healers too (for XY healing). Being pyro could also help with Bennett synergy, pyro resonance (which benefits XY), vape, vv, etc. Besides anemo (Xiao), autumn dps (pyro/cryo/electro/hydro) will also benefit from XY's 16s of vv app (can be 16+ seconds due to vv lasting for several seconds).
So Gaming tends to fit most of these characteristics above.
As XY can be the vv character, it may be redundant to put XY and Kazuha on the same team. In the end it boils down to: does one want plunge buff from XY or dmg bonus from Kazuha (as both provide vv)? And if one chooses XY, is Kazuha's dmg bonus buff large enough to compensate for not using Bennett or Furina? The reason why Furina is used (besides enabling vape for plunges without needing NA unlike Yelan/XQ) is because her Q also provides an insane amount of dmg bonus, which can be substituted for Kazuha's harmony buff. So one can use Kazuha on the other abyss team.
Obviously if the dps is like Xiao (anemo) and does not use the autumn dmg bonus from Kazuha, then Xianyun is the way to go. But for say catalyst or bow dps like Wrio or Ganyu, perhaps Kazuha is the way to go.
So what is the baseline dps one can achieve with a Bennett C6, XY, Furina team? Suppose the high plunge multiplier is 400% (some characters may be more or less at lv10, btw plunge also incentivizes triple crowning as now the plunge from AA matters and can be used). Assume 61.6% pyro dmg (goblet + 2p). Bennett will give 15% more with Q. Suppose XY is at 3k atk (confers 200% of atk as plunge buff, so effectively 200% more scaling). We say 3k atk instead of 5k atk as if XY is the only anemo an ER sands could help her energy. If XY is in say a Xiao triple anemo team (with Faruzan) then perhaps XY may not need ER sands and can vouch for ATK sands and reach higher than 3k atk. But for general non-Xiao dps, we will assume 3k atk XY.
We assume 100% uptime on XY burst, Bennett burst. We do not assume Furina burst (as that would need 3 bursts on 100% uptime, which could be on the more cope side in practice). Ideally, the dps character (4th slot) can do dps outside of burst (like AA/E dps) for flexibility and efficiency. We assume 8 plunges in XY's burst (has 18s cd), and we assume each plunge vapes. We assume 0 EM. The rotation is 18s, and the dps is $$x = 8*1.5*(1.616+.15)*(400+200)/18 = 706.4$$ This is aoe (plunge) and quite high.
What does this mean for future pyro dps? Let's consider Arlecchino. First as plunge dps is aoe, we would need Arlecchino to do aoe dps (equal to or larger than plunge aoe) or else an Arlecchino dragonstrike / plunging team will just do more damage and have more aoe. Second Arlecchino's 'normal' way of doing damage (whatever that may be) should have higher mv than above (700%) or else again the Arlecchino plunging team will be her best dps team. In other words, Arlecchino's 'usual' way of doing damage ought to have more scaling/mv than plunging, and since high plunge is around 400% scaling, that means the mv will need to be high.
There are two main ways to do this: either the actual scaling needs to be higher than 400% or if it is split into multiple parts (like Ayaka Q), it better attack quite frequently and fast (also to compensate for how a 400-600% plunge can vape for a larger nuke). A 600% plunge (with XY buff) that vapes has 900% scaling (per plunge, every 1-1.5s). Something that is split in multiple parts would need total scaling of at least 900% or more (every 1-1.5s) if it wants to compete with a default Arlecchino dragonstrike team. For example, Ayaka's Q is 20 hits each around 200% scaling for a total of around 4000% scaling within 5s. Something like 2000% scaling over 2-3s would be similar.