Here we define sub-dps as off field damage. Various characters provide off field damage. Some proc on NA only like Beidou, XQ. Others proc on hit: Raiden E, Albedo. Others will automatically hit: Yae, XL, Morax, Fischl, Venti, Kazuha, etc. Interestingly, Raiden's burst NA do not proc Beidou's Q but do proc XQ's Q.
Of off field damage dealers: Yae, Raiden, Albedo, etc. have 100% uptime. Usually for sub-dps there is a limit/'wavelength'/internal cd (not to be confused with the icd from gauge theory) to how frequent the damage is dealt. If x damage is dealt per hit, then dps is $$z = x \frac{duration}{icd} \frac{1}{\max(cooldown, duration)}$$ or x times the number of hits within the duration divided by the cooldown of the skill. Note if the cooldown is less than the duration (ie 100% uptime) then the above simplifies to x/icd.
Of particular interest is the completely automatic team where you don't do anything at all and let the sub-dps do all the damage. This is referring to a Yae-Fischl-Albedo type team. Each of these characters has 100% uptime (or close to it), and they provide automatic sub-dps. Yae and Fischl automatically hit, and Albedo's E will proc when they hit.
The sub-dps from Yae and Albedo is $$z = 488.0 + 239.8 = 727.8$$
The main use of electro application is for quicken and quickbloom teams. The premier electro sub-dps are Yae, Raiden, and Kuki. Each offers their own advantages. Yae provides the most off-field damage and has 100% uptime (with 14s duration), but the turrets hit single targets and aiming is random. Raiden provides 100% uptime (25s duration) and aoe coordinated attack, but scaling is lower. Kuki provides healing, 100% uptime at C2 (15s duration), and does aoe application centered around the character, but scaling is also low.
Fischl is just a worse version of Yae.
Each of these electro units is useful in different teams, particular with their dendro partner (Alhaitham or Nahida). For Alhaitham, he may prefer Yae or Kuki depending on team: Alhaitham-Yae-Nahida-Kokomi or Alhaitham-Nahida-Kuki-Yelan. On the other hand, Nahida's off-field sub-dps synergizes well with Raiden in a dendro Rational team.
Off field application can be done via E or Q. It can be 100% uptime or part time.
E sub-dps with 100% uptime (or near 100% uptime):
E sub-dps with 100% uptime are usually broken as they offer free elemental application with no energy cost. It was only until 3.5 when Dehya released that the game finally got a pyro applicator via E with almost 100% uptime.
Kokomi does do hydro sub-dps on her E, but she faces the same situation as Fischl: to make it close to 100% uptime, she needs to use her Q to reset her E. But given Kokomi's 70 energy cost burst, this is not that convenient to achieve. By contrast, Dehya's C2 will allow recasting the E to extend the 12s to 18s out of 20s.
Q sub-dps (can be part time)
The reason for absence of cryo sub-dps (basically a cryo Nahida or cryo Dehya) is that such a unit could be broken for pyro melt teams, if one could consistently apply cryo over a long period of time free of energy cost. Although Ganyu's Q does apply cryo, it has ICD, costs energy, and is too slow/not fully reliable. Rosaria's Q and Kaeya's Q also have ICD on cryo application, besides the energy cost.
A cryo Yelan/XQ (on Q) would be good for pyro melt, but would still cost energy, and with the introduction of dendro would not be as much of an upgrade given that hydro can enable bloom/burgeon/hyperbloom, whereas cryo application cannot.
For example, even if a cryo Nahida or cryo Yelan released, that would not replace Yelan/XQ on say a Cyno quickbloom team, given cryo just does not interact with dendro.
What would a cryo applicator be needed for? Melt, superconduct, and freeze. Of these melt needs a good cryo applicator the most. Superconduct lasts for 12s and does not need as much cryo app. Freeze can just use an off-field hydro applicator (plenty of these) and does not care as much about icd either given enemies are frozen for periods of time (duration of freeze state appears to depend on certain gauge mechanics). Freeze teams function fine as cryo on-field + hydro sub-dps. The reverse with hydro on-field + cryo sub-dps is a bit harder given the limited number of cryo E sub-dps (most cryo sub-dps require Q and energy cost).
The role of an Anemo E sub-dps would be to apply vv res shred for long periods of time (say for someone like Cyno with long burst time). The since VV res shred lasts for 10s, one can get by with 8 seconds of anemo app, to extend to 18s (provided by both Venti/Kazuha Q). But an Anemo E sub-dps that functions like Raiden or Nahida E, with 25s duration and an aoe proc would be very convenient for Cyno. As for the frequency of the proc, given vv shred lasts 10s, something like Nahida's E where it procs every 2.5s could suffice, though there is nothing wrong if it procs as frequently as Raiden's 0.9s E.