Is your feature request related to a problem? Please describe.
Sort of... If I use the "portal" material for a horizontal gate I get the end portal, but vertical gates use the nether portal. This is good. I just wish I could have the end portal texture but vertical.

Describe the solution you'd like
To achieve this, a new material could be added using the End Gateway. This block looks like the end portal, just flipped on its side, thus solving the problem.

This would be cool but I'm not sure if it would be possible to do. Never tried it but I think the end gateway doesn't fill a whole block so it would look like portal bars if it's even possible to have multiple of them side by side without them breaking. I will get it a try when I can though.

I'm not sure if it helps, it probably doesn't, but I've attached a screenshot of a redesigned end portal room from a server I admin. It uses end gateways to make a vertical portal somewhat reminiscent of a nether portal. Granted, there is a 3x1 end portal under the gateway blocks because the gateway blocks have no native functionality in the overworld.

Took a bit of work but it seems to work just fine, try this when you can: https://ci.codemc.io/job/ChanceSD/job/AncientGates/25/
You can set any portal to use end gateways with /gate setmaterial id ENDGATEWAY
I will probably upload it to Spigot soon if there are no issues.