I’d love to see some documentation from the '60’s where making the Klingons look more alien was actually discussed. A lot of production memos and such still exist. It’s not that the capability wasn’t there, (look at the Tellarites or the Andorians) but the expense of doing a large group of Klingons would have wreaked havoc with the budget.
I don’t know if such a discussion actually occurred. But you know they did not have much budget for TOS. You also know there is no way back in the 1960s they could have made the Klingons look as they do in nextgen.
TNG tweaked the Romulan look enough that you have to wonder how Spock can walk around on Romulus unnoticed. 😉
I thought it was stupid to do. Long before the tweak, it was established that Vulcans and Romulans looked alike and had a shared ancestry. Therefore, it makes no sense to tweak the look of the Romulans without tweaking the look of the Vulcans. The look of the Romulans and Vulcans were fine, no need to tweak them.