Not GL himself, aliens beings and environmenta in a GL show? Probably. i know they are not going to do an all CGI suit again, no matter the medium.
Aliens and environments can be done practically, and so that doesn't need to be that expensive. But GL's powers are ALL CGI all the time.
Unless it's not, unless you do a GL whose powers are done, for the most part, practically. But that becomes a significantly different character, kind of like Firestorm that only throws fireballs.
I really don't see why people keep saying GL would be so expensive when we see Firestorm done regularly and within budget, not to mention other things like King Shark and Gorilla Grodd which are far more expensive to pull off then just putting a green aura around a flying character.
Also no one knows for sure yet if Superman is only allowed on Supergirl, we could very well see him crossing over onto the other shows in some capacity.
Well, to me, the character they show is a pale, pale reflection of Firestorm and even that is also only around for two minutes tops, assuming no one else is doing any notable cgi in the episode.
And I think you make a good comparison, the two minutes of King Shark/Gorilla Grodd is about the same as putting a green aura around a flying character. The thing is, people want a whole lot more out of Green Lantern than just him flying around glowing for a minute.
I think once you try to add CGI to a real person, you'll realize that adding a CGI creature to a dark scene with no interaction with real world objects is a LOT easier than adding glowing and lighting to a real character, or creating a cgi person that works well. I think the only way someone can feel Grodd is harder than "just a green aura" is without knowing what's involved in having CGI, light especially, interact with a real person. Even flight isn't "easy" to make look like its really happening in the scene.
Flarrow can't even use Suicide Squad characters anymore. So, no, they can't use Superman and Batman, and nothing's changed. They can't even mention Supergirl on the show, so far. There's no reason to think they can mention SuperMAN now just because Supergirl is on the same channel.
They can fully render planes, gorillas, sharks, green martians, white martians. They can make a green light be fist-shaped. And his aura wouldn't be different than the vibrating effect for Reverse-Flash or the blue lightning glow for Zoom like in my avatar.
I wouldn't say they couldn't do GL at all. I said that two minutes would wreck the budget. They could probably get a minute of GL in just fine, like they've done a minute of Grodd or King Shark. Doing blurs is a lot easier, in my experience, as I can do something similar at home on middling software.