I think you might be thinking of this, truly one of the crimes of our lifetimes. Anyway, I cannot fully understand the idea without playing it, but it seems... exceptionally convoluted for a DOT. To the point where I'd say it's not just 'a DOT', but more like the job's identity. Reading this confuses me more than the old HW card effects. Like, you would have to remove the cards entirely to free up the mental load required for this thing, and even then, I'd expect SE would say 'no that is too complex to juggle alongside healing' (and in this case I'd probably agree), so it'd go on Time Mage DPS as 'Singularity' or something. I'm 100% onboard with nuking Combust from AST because it's just boring as hell, if said nuking nets us additional gameplay in alternative parts of the kit more aligned with the identity of the class (ie, the cards), because the one thing Combust COULD have had going for it was integration with the Time Magic stuff like Time Dilation/Celestial Opposition, but now they're gone, so is Combust's reason to exist IMO. If they want to have some kind of... 'this move deals big damage, but in X seconds after you use it' to replace it, that's fine too
edit: re: Time Mage being a potential better fit. Casting Singularity begins the building phase, and you get two balls floating round your head. You'd have two sets of skills, with varying effects, DOTs, direct damage, whatever. One set, let's call it Matter and Antimatter. As you use skills from each of these two sets of skills, you'd do your rotation, but also add matter and antimatter to the two Singularities in varying amounts. You'd have to react on the fly to the levels of each, a bit like RDM mana bars I suppose, but I'd expect the builders to have more unpredictable gain amounts. For example, maybe only one of the two sides has a DOT that fills that side's bar per tick, the other has to be covered with extra casts of the filler spell to compensate. The idea would be, you'd be trying to keep these two Singularities roughly equal in size as with your idea, such that one doesn't just eat the other. Casting Singularity again would allow you to force the two to collide, causing big damage because mixing matter and antimatter does that, and that'd be your burst damage for raidbuffs. Rather than RDM's 50/50 spend, this would be 'consumes whole gauge, scales damage based on amount consumed', and if you imbalance the bars, rather than halving your gain till it's corrected, you'd get some kind of dud result that does big damage, just not as big as it could have been (a Celestial Revolution of sorts, even the name kind of fits). Maybe it sounds all too similar to RDM, but I'd like to think that having some actual Time Mage stuff in there to manipulate how fast you gain Matter/Antimatter (eg, Matter's the side with a DOT, having a way to extend that DOT, or instantly collapse it to get the benefit immediately to emergency-balance the meters) would help set it apart at least a little bit