You said you couldn't use power names.
But in any case, "level 5" and "Fighter" are both covered by the OGL, which likewise has plenty of examples of things usable "once per day". And as we have discussed mechanics can't be covered by copyright.
There are a lot of things WotC do own, and there's even more that someone would need to be very careful with. But it's equally a mistake to ascribe them ownership of everything.
This has been discussed in the past. I'm not aware of anyone turning up any patents on tabletop RPG design.
When OSRIC was first published this very issue was raised. It turned out that the designers had indeed taken extensive legal advice before they published because of this very concern. They were very certain they were in the clear, and that opened the floodgates for everyone else.
Now, of course, that hasn't been challenged by Hasbro in court, which means it can't be certain. But if you're argument rests on "you can't afford to fight" then we're not actually talking about the legality of a retroclone, but rather Hasbro's willingness to bully.