Since Sage first released, I've constantly criticized how bad Toxikon and Addersting are as a mechanic, and I've only doubled down on that since. But I actually, finally thought of a way you could keep Toxikon as a GCD with the same potency as Dosis and actually be a functioning part of Sage's kit. Here's the idea:
Throw what you know about Eukrasia out of the window. Because in this example, the three Eukrasian spells you're familiar with do not exist. Instead, Eukrasian Dosis is an attack that has the same potency as your current form of Dosis, only Eurkasia now comes with a hefty MP cost--something like 1200 maybe, or perhaps even more. What changes is that it applies the Eukrasian Diagnosis barrier you're familiar with to your Kardia target in addition to dealing the same amount of damage as an ordinary Dosis cast. Now, if that barrier breaks, then you generate 1 Addersting.
This is where Toxikon comes in, because it's still the same damage as Dosis, but because we didn't have to lose any DPS by casting a targeted heal, it remains neutral with casting Dosis over and over. The ticket here is that Toxikon has no MP cost, and consuming Addersting now restores your MP. In other words, you spend a large chunk of MP to protect the target of your Kardia with a barrier, and if that barrier breaks, you get a resource that's used for mobility and recovering a portion of the MP cost of applying the barrier in the first place. But, it's still more MP even if you get the Addersting proc than simply casting Dosis, and if the barrier doesn't break, it's a large loss of MP.
The problem with this is that Sage has so many OGCD healing resources on ridiculously short cooldowns that it raises the question of "when would I even want to spend the MP for that anyway?" So in order for this to work, you'd need to drastically nerf Sage's ability to heal through weaving at every given opportunity and make protecting with Eukrasian Dosis' barrier the most consistently available option to you.
I still would rather Toxikon be a directly offensive resource than an MP management tool, but I wouldn't hate that if the landscape for healing reflected the desire to rely on that as your first response to damage taken, with other options being more limited or suboptimal.