The gemara says that the preferred vegetable for maror is chasah — lettuce. The problem is that the lettuces sold today aren’t mar, bitter.
One could try to find wild lettuce. The lettuce that was native to Israel is prickly lettuce, Lactuca serriola. Of the wild species of lettuce, the closest to Lactuca sativa, what humanity bred lettuce into. Of the various types of cultivated lettuce, Romaine — or cur lettuce, as it is called in most of the world — is the closest to wild. Which explains why romaine is generally used. But still, it is hard to call it bitter.
And in fact, prickly lettuce would continue one of the themes discussed about the qorban Pesach, the point of taking a lamb, something the Egyptians considered sacred. Or, as per the Avudraham [Abudarham], the constellation of Ares was sacred, and killing the sheep it represents during the month of Ares would really be seen as blasphemy. But whether the primary Egyptian concept was the constellation or the animal, the qorban is a denial of their religion.
The Egyptian god of male sexual potency, Min (or Khum) was worshiped using prickly lettuce. The plants are tall and have a white sap, so the symbolism for male fertility is kind of blatant. During the New Kingdom (16th-11th cent BCE), the Pharoah would throw the seeds of these plants into the Nile as part of the coronation ceremony. By most chronologies would include the Par’oh of yetzi’as Mitzrayim.
Min’s festival was at grain harvest time, each spring — Pesach-time.
I would therefore think there is poetic justice in placing a sheep on Min’s sacred plant, and turning it into a qorban.
It is that white sap which gives lettuce it’s name; “lettuce” is a derivative of the root “lact-“, milk (as in lactose, lactate, etc…). And to the American eye, prickly lettuce resembles very tall dandelion, a/k/a milkweed plants. (Complete with yellow and and those cloud of seeds we used to call “blowies” as kids.)
R Yochanan famously says that charoses is in memory of the mortar. Abayei disagrees and says it’s in memory of the apple trees, presumably those under which the Jewish women birthed their children, hiding them from Par’oh’s decree of death. (Pesachim 116a) But a little earlier, in another discussion, R’ Ami says it’s to take care of “kappa“. Rashi defines “kappa” to be something harsh in the sap of the maror, Tosafos say it’s a bug.
(By the way, charoses lovers, Rashi’s explanation would seem to me to imply that while one should not be eating so much charoses as to bury the tast of the maror, there should be enough charoses on it to somehow take some of the harshness off the bitterness. Similarly, Tosafos asks how charoses, which we hold is not a mitzvah, could be allowed to interfere with the mitzvah of maror. They answer that because of kappa the mitzvah itself was made as including the dipping; the dip is not add-on to the mitzvah of maror. [114a “af al pi“] So, even though they could have said that once the charoses gets the bugs to drop off, they can be shaken off, this question implies that Tosafos did expect it to be eaten. And the Kaf haChaim [475:32] points out that the Beis Yoseif / SA and the Maharil do not even mention shaking the charoses off as part of koreikh at all. Totally changing how one views later rulings about needing to shake off charoses — apparently some should be left on, just not enough to overwhelm the maror. [As Rav Ovadia Yosef rules in Chazon Ovadia, pp 89, 101; Yalqut Yosef, Moadim pg 405.] I think we’re living through a hyper-correction after a generation or two omitted the Shulchan Arukh’s צריך לנער החרוסת מעליו altogether. But back to the point, Rashi would require eating some charoses, so as to avoid being poisoned.)
In light of Rashi’s position, it is interesting that prickly lettuce (and dandelion) sap is a mild soporific and a something if a diuretic, it can even make one quesy if not actually nautiaus. It’s also an alkaloid, so an acid, like the vinegar generally associated with R’ Ami’s position, would neutralize it. Which would explain why apple or pomegranate vinegar appear in many charoses recipes.
In practice, will I use dandelion at my seder? Well, I own and occasionally walk 2 dogs. I know what gets on the local dandelion leaves. No thank you. But kidding aside, I have romaine and endive, like normal people. Then I have horseradish, for nostalgia, even though it lacks the gemara’s requirement that maror be a leaf. (Apparently it’s a legacy of having ancestors who lived in a region where there were no leafy vegetables available yet this time of year.) So, I keep the horseradish eating separate, so that it doesn’t hide the taste of the preferred marror.