Idk about the whole 'remove DOT and deal 50% of the damage it has already done, again' it seems kinda wonky to me. I think something more functional/better feeling would be something we've already got in the game, Iron Jaws. If Katarrh extends the duration of Myalgia (at the cost of 70 potency vs Dosis), I think it'd probably feel better that way. Since as it stands, if you were to be forced to use Katarrh and consume Myalgia at 9s (you need to heal, fight design, etc), you are punished both by losing an extra tick of the DOT, AND by the compiled damage being lower, whereas extending the DOT would only be 'you lost a tick'. Then again, maybe it can be argued that IJ is kinda button bloat and only justifies it's existence by the fact it extends two DOTs in one GCD, so... I think trying to come up with a way to make the DOT have some interplay with the rest of the class's gameplay is a good idea, but I'm not sold on if this is the way to go about it
Hmm, if SE's engine worked better, and we had something like WOW (haste stat increases speed of DOT ticks, they don't have to be bound to strict server tick cycle, etc), you could have Katarrh as 'increases the speed at which Myalgia deals damage by 50%, for 10 seconds' or such, to make it an upkeep buff you want to alternate with Dosis occasionally. Instead we'd have to use a workaround like 'Katarrh applies Katarrh to enemy' and 'If enemy has Katarrh, Myalgia's damage is increased by X% upon application', which is, lets be frank, way more boring. I dunno I think people will find the timing aspect to be super annoying more than fun, due to how fight mechanic/heal-pump-moment timings line up. We can already see people getting annoyed by how SE keeps making ASTs do their cards during a complex mechanic because 'we have to challenge the players during their 2min burst to test their skill' or whatever
What if you go back further design wise, and look at tying the DOT to the healing, rather than another damage tool? For example, if Katarrh adds a new buff to the SGE that allows a portion of damage caused by Myalgia to transfer as healing through Kardia for X number of ticks, or as a barrier instead of pure healing? Of course, this would have to be carefully balanced re: AOE, I see a Eukrasian Paroxysm that applies the DOT in an AOE. Although, if we were to say, it transfers 50% of the DOT damage, each time it ticks, that's 35p per enemy in the pull, right? So in a 10 mob pull, you'd still have the player ticking for less HOT than a simple Regen+Medica2 combo (150+250), and WAR gets to do that 400p, per mob, 3 times every 25s, so maybe it's fine