I think that the best option is to suspect Bulbasaur.
The main justification behind banning Chlorophyll or Heat Rock instead of just banning Bulbasaur would be to nerf manual Cherubi sun as well. However, manual Cherubi sun carries numerous disadvantages compared to the suspect-worthy manual Bulbasaur sun (and is practically incomparable to the banworthy Drought Cherubi sun) to a degree where I don't think they should be treated as equivalent threats. While they share a core identity of Weather Ball Chlorophyll sweeper, Bulbasaur is much better positioned to take advantage of its support in the current metagame than Cherubi is:
1) setup opportunities. Other than its slightly higher special bulk with a spread of 21/12/13 bulk to hit 15 spA, it has a much more valuable defensive typing that grants it setup opportunities on key mons in Mareanie, Oddish, and non-Fire Punch Timburr, as well as some lesser threats like Croagunk. It still has the bulk to set up on Mudbray and Psychic Spritzee, and can often Giga Drain to escape priority range, leaving LO Diglett (which often chooses to run a utility item instead) as the only mon that Cherubi can consistently set up on and Bulbasaur cannot. The Poison typing also grants Bulbasaur an important resistance to Mach Punch and a neutrality to First Impression, the latter of which means it doesn't have to waste valuable sun turns on Protect.
2) offensive pressure. Sludge Bomb helps Bulbasaur break through Ponyta, which walls Cherubi completely, and makes it much harder for Spritzee to stall out sun turns. More importantly, it allows Bulbasaur to continue to act as a major threat after sun has ended. Once sun runs out, Cherubi is effectively negated as a threat; it's slow, frail, and only has Grass-type coverage. On the other hand, Sludge Bomb ensures that Bulbasaur still has the power to OHKO most non-Eviolite holders at +2 and at least threaten various mons at +0. Bulbasaur also carries a better speed tier for Mudbray, Mareanie, and speed tying Vullaby.
Not only do these advantages make Bulbasaur the generally better sweeper, but they also directly explain why Bulbasaur wouldn't be as badly impacted by sun being nerfed from Drought to manual setters. The biggest disadvantage to manual sun is that it's drastically more difficult to find opportunities to set up, especially in a way for the abusers to actually take advantage of what's on the field, than simply switching Vulpix in; Bulbasaur's greater number of setup opportunities makes this less challenging. Manual sun also offers less turns on average for its abusers to utilize, seeing how it takes a turn to set up, which is partly mitigated by Bulbasaur still remaining a serious threat once it ends. Dedicated manual sun tends to require a lot more suboptimal sets to support its abusers, and Bulbasaur can help to cover this by offering significant general utility where Cherubi cannot. Despite the means of support having been massively nerfed, Bulbasaur can still consistently act as a top tier threat.
As for Chlorophyll spam, I don't think this is a major consideration when Cherubi/Oddish still share a majority of their switchin opportunities (Oddish has a bit more) but not their checks, meaning they don't synergize defensively and don't break checks for each other very well. (
I addressed why I and most of the council believed Cherubi and Oddish to be independent issues offensively when we chose to ban Cherubi.) Even if they did synergize defensively, stalling out sun turns is already the primary counterplay to manual sun, and having to switch between the two would make this a lot easier.
This isn't to say that manual Cherubi sun wouldn't still be good, I just can't see it being anywhere near able to establish the presence that Bulbasaur sun has established in LCPL. Right now, I do not consider it to be strong enough to warrant a suspect, but even if it was, I think that it's different enough from Bulbasaur sun to warrant a separate suspect down the road instead of an immediate one. I've seen a couple comparisons drawn between the timing of the suspect versus the DLC and what happened with SM Vullaby, but the whole reason behind denying the SM Vullaby suspect was that it would be a huge change to the metagame with no chance afterward to develop it as a current gen, whereas we'll still be developing and tiering the metagame regularly after the DLC.
There is nothing inherently broken about Chlorophyll or sun as a whole. It doesn't make sense to claim that the issue lies in mons with doubled speed, when other weather forms are mechanically similar; or with sun-boosted attacks, when Chlorophyll users other than Bulbasaur would be comfortably balanced with manual sun; or even with Weather Ball + Chlorophyll in particular, when manual sun was barely on the cusp of viability in SM. The issue lies purely in specific abusers - in this case Bulbasaur - being too strong, in the current iteration of the metagame with its currently available tools, so we shouldn't be treating sun as if it were uniquely cheesy or problematic.
On the topic of basing our decision on future drops, I disagree with it simply on the grounds that there isn't really any benefit to doing so. If Bellsprout does get released, we can immediately switch to a lower collateral ban with zero extra steps and no further repercussion; I don't see why we should base our decisions on unreleased information when we should be making the most sensible decision for the current metagame that we're playing. This is why we chose to ban Cherubi pre-home. Banning Cherubi drastically decreased the complaints against sun to be a small minority, and to my understanding, most of those were citing that Cherubi autosun hadn't warranted a ban either (Heysup, MK007). The potentially problematic ban, if we do believe Bulbasaur to be broken, isn't Cherubi's ban, but rather unbanning it and banning Vulpix instead on Bulbasaur's reintroduction, since it may not have been enough to solve the issue.
With the understanding that Cherubi sun isn't broken, or at least is an archetype that warrants a separate look, I think that suspecting Bulbasaur makes the most sense. It's the most direct approach, and the only collateral would be fringe non-sun Bulbasaur sets that already struggle to compete with Oddish and Budew (it does get Knock Off). Banning Chlorophyll would remove an otherwise viable archetype in Cherubi sun completely by rendering Cherubi illegal, while still reducing Bulbasaur to a barely considerable level of viability. Banning Heat Rock would in my opinion carry even more collateral than Chlorophyll in terms of metagame influence, as cutting the number of abusable turns from 6 to 3 would likely render supportive sun setting all but unviable. This would remove firespam as an archetype and just leave self-setters, which is probably just Ponyta (unless you count self-setting Bulbasaur, but I don't think that what's effectively a 17 SpA Agility sweeper on a 4 turn timer would be justifiable). It's worth noting that banning Heat Rock at least wouldn't outright render Cherubi illegal, though.
[33.3 - 42.8% // guaranteed 3HKO after Stealth Rock]
