Sure it could.
Illidan was a known sorcerer, who is also the one who defined Demon Hunters altogether. Demon Hunters ultimately train in the use of fel magic. And WC3 already depicted them with a fully ranged attack in demon form.
It just as much makes sense if they had a spellcaster spec. Blizzard simply didn't choose to pursue it. Doesn't mean it doesn't fit the concept. For example, Warlocks didn't get a full 'Summoner' spec until Legion revamp, even if Demonology spec always focused on the use of just one demon, or even no demons while empowering the Warlock. It doesn't mean that the Summoner style of gameplay 'didn't fit the concept of the Warlock'; it was just not fully realized until Legion.
A few years back, would you say melee doesn't fit a Hunter's concept? Just because Hunters didn't have a proper melee spec until Legion doesn't mean melee gameplay didn't fit the Hunter concept. And let's be clear, Hunters never having a proper melee representation is the same right now as Demon Hunters not having a proper ranged representation. It doesn't mean it's outside of the class concept, it just means the gameplay hasn't been represented in such a way at the moment.
And on the topic of what fits a concept, anything can also be added. Look at how Paladins are definitively attached to the concept of using Shields, or Shamans being tied to the use of Totems. These were never part of the original WC3 depiction of Paladins and Shamans, they were introduced from other sources. So if we're talking about spellcasting Demon Hunters, then it would be no different bridging in spellcasting concepts from other sources such as what we already have; Fel-based demonic Warlocks, Anti-magic specialists like Spellbreakers and Arcane-trained Sorcerers like Illidan. It's all about bridging concepts into a WoW class.
I mean look at it this way - there was never a tanking Demon Hunter who used Vengeance Form and wielding the half-blade Warglaives before Legion. Demon Hunters were not associated with tanking either; their evasion exists completely in the Rogue class which is also a melee combatant that is not a tank. The Vengeance concept was never a part of Illidan's core concept, yet you willingly accept that it fits in the Demon Hunter Concept even though it never existed. Curious that, no?