Blue
Ravenous Bugblatter Beast of Traal
Since I've run games with 2 or even 1 person without house rules I'm not sure what the concern is. The current edition is pretty forgiving if you want it to be, you just have to tailor to the players.
But if you see it as a problem my list would be
Unsure where you are getting "concern" and "problem" from - I explictly said that 5e can do it in the very first sentence, and reinforced that later.
That does not stop one from wondering if there are additional things out there. It's like how we read UA and buy new books to see new options. For example I'm having a blast playing an Order cleric from Ravnica even though I didn't think there was a problem with the cleric domains in the PHB.