I would love to see Jason Blood show up on Constantine, I could see him being another Mage like Constantine only he has been alive since King Arther and is more clean cut looking. He and Constantine must stop some threat like Klarion or Morgan Le Fay and together they use their knowledge of magic but in the end to save them Jason must call upon Etrigan and transform. After Etrigan quickly ends things it takes a few minutes to get him to return Jason's form.

It can be easily done. They did it all the time with effects and computers on Buffy and Angel. There was a main character who was a human who transformed into a giant demon all the time, took two actors and computer effects. The effects could get expensive for a show that the studio might not pour money into but that would be why the Demon isn't shown until the end and why we focus on Jason.

For the design they can have Etrigan in his New 52 knight armor so it takes less makeup and therefore less money. I'd love to see Jason Blood's past explored as well and how he has lived for a very long time.

I sure hope the find a way to keep Constantine going because I'd love to see Jason Blood/Etrigan, the Spectre, Eclipso, Zatanna, Ressurection Man maybe, and others on the show.

Here are examples of the level of effects possible now, or at least years ago (so now should be better for less):

The armor can cover him (like in the New 52) from neck to toe and only reveal his demonic head and hands. That should also keep the cost down and the armor could help the actor look bulkier than he would actually be.