An easy-out may work, but typically, you'd have to cut or grind part of it off since the taper would make it too long to actually get a grip.
If you are very careful, you can try to peel it out. Depending on how rotten it is, you may need to take a hacksaw blade to score the inside, then pry it out of there.
Threading it wouldn't likely work - it's a tapered thread, and it would take a special tap to do it. Not sure if you can find them that would work in that shallow of a hole. It would require drilling out the hole, then something similar to a bottoming tap, but a tapered one for the IPS threads.
You may want to spray some PB Blaster or another penetrating oil in there and let it sit for a hour or so before you try to get it out.