The latter. When writing a somewhat standard implementation people expect redundant special forms (like both if and cond) so there are more than you think. OTOH you can implement some in terms of the others so maybe there aren’t as many as one might thing.

Also, of course, in 3Lisp you can run code in your interpreter and so define new control structures and such. Turns out there aren’t many interesting ones and they have mostly already been thought of.

One new control structure that didn’t need to modify its own interpreter was method combinators. Turns out they mainly useful for unpredictable behavior, except in very simple cases like :before and :after.