Start from the bottom:
Now consider the 3-letter words beginning with P.
Now in the top right we have limited possibilities:
Now there's lots of other stuff to be filled in around those long words on the left, and some other deductions which lead us all the way to the centre.
Now only four 5-letter words remain to be used:
Solved independently of @El-Guest (kudos for being first to find a solution to the crossword), with two differences:
The resulting letters in gold boxes are:
Rearranging these letters can produce the following 4 country names:
What is the connection between these 4 countries? Well, there are potentially several - but one lends itself naturally to an ...
Partial answer because it’s now past my bedtime. Finished the crossword but I don’t think this solution is unique (in the upper right corner, specifically) so I don’t know if I have the right letters. As such, though:
The answer to the question of what your crossword is missing is:
A lot of credit for solving the initial crossword and wordsearch needs to go to @BeastlyGerbil (go upvote their answer!) with helpful contributions in comments from @Anthroman, @LannyStrack, @teedyay and @oAlt. The solution details for those are listed in full in BG's answer, with the finished ...
@Stiv found the final answer, go see his answer for an explanation! The thing you are missing in your crossword is
Now clearly there is something a bit odd about this crossword...
All the words follow a pattern
See Stivs answer for the final step!