I think that depends on what you mean by "iconic." The word suggests "representative of D&D." It isn't synonymous with "D&D product identity." The red dragon is the iconic monster of Dungeons and Dragons, but it's not product identity.Against what I've seen repeated here on EnWorld a few times, I actually would be surprised if the Basic Game was OGL'd. They've continually referred to the monster and DM content in the Basic game as their 'most iconic'. I would be surprised if their 'most iconic' stuff is the stuff they give for others to make with. Much of their most iconic stuff didn't make it into the OGL, for instance.
Thaumaturge.
According to d20srd.org, the list of product identity monsters (at least in the original MM) is limited to:
- Beholder
- Gauth
- Carrion crawler
- Displacer beast
- Githyanki
- Githzerai
- Kuo-toa
- Mind flayer
- Slaad
- Umber hulk
- Yuan-ti
I think you could make a Basic D&D that would remain "iconic" without including any of these guys. Or they could just do what the 3E folks did and hold back a handful of product-identity monsters, while putting everything else under the OGL.
But you're probably right. I don't really expect Wizards to go OGL with any part of 5E. More likely, we will be looking at some breed of GSL, hopefully less restrictive than the original.