GD already has a few fairly pronounced telegraphs. Zarthuzellan, Ekett’zul, and Bolvar come to mind.
In my nonexistent “ideal” ARPG locked away in the grey matter of my head, something somewhere between what GD has and what Wolcen has would be optimal. GD doesn’t need many telegraphs because its AI is fairly simplistic - most simply run up to the player and start meleeing them over and over. True Caster AI is few and far between in Grim Dawn, but I would LOVE to see more of it. For casters which have larger intervals between casts to remain dangerous from afar, their “spells” need to be quite potent. However, in order for that potency to be “fair”, the cast time and cast point (DotA term for the point at which a spell actually comes out in an animation) need to be somewhat obvious to players. This is where telegraphs come in to play.
I don’t know if something as obvious as Wolcen’s ground markers is actually necessary, but I wouldn’t be so hasty to outright rule the idea out, either. I think it’s more nuanced and situational than could be described by anything catch-all. Look to PoE - the Shavronne (/Brutus) fight is pretty fair but nonetheless dangerous at high levels, because even though there are a lot of dangerous spots on the map (the exploding books), the AoE of their danger is well described to players. On the other hand, Doedre (the fat version of Doedre) I find to be much more difficult because it’s somewhat unclear with PoE’s camera angle where her projectile attacks will wind up.
There are times in Wolcen when I wished there were MORE ground-geometry telegraphs (the Edric fight) and there were other times when I wished there were fewer (the Lambach fight). Striking that middle ground is key.
Edit: I vehemently disagree with the immersion argument. Telegraphs such as Wolcen’s ground geometry are an example of where we, players looking down on our characters in an isometric view, would have less understanding of the characters’ surroundings than they themselves would. In the screenshot in the OP, that character would have a better view of the attack its enemy was throwing out, and so inherently the most immersive thing would be for us to know what the character knows. Without that mutual understanding, immersion is lost.
TL;DR, ground geometry telegraphs are simply an optical translation between two angles in an isometric scene.