There is actually very little WOTC can claim as copyright, trademark is much more obvious as to what you can't use. But hit points, to hit, damage, saves, abilities, ability modifier, feats, skills, etc... are not copyrighted and cannot be. Even Wizards, thief, warrior, fighter, barbarian, etc... cannot be copyrighted.
Publishing 4E compatible stuff via the OGL opens things up a bit more as well.
Staying away from specific formats, names unique to D&D, such as Mordenkainen, Bigby, etc..., since those are copyrightable, and I believe still are, are a good start to avoiding copyright infringement.
Not using other phraseology identifiable to 4E would be wise too, like you wouldn't want to refer to the "roles" the character plays, you don't want to be using their monster stat blocks, or refer to their books.
You can say the monster to fight is a "spherically shaped creature, with many eyes on the end of sticks", and the reader will have to figure out what creature you mean and look it up in their MM, or call it the "Many Eyed Fiend" and write up your own interpretation of it, but don't use the same format WOTC does.
So going the copyright route is very doable. Possibly even more so if you use the 3E OGL in a manner similar to how OSRIC was done.