4th Gen Help with completing team?

I'm trying to use Pokemon that I haven't used much, if at all, in my next Pokemon Platinum playthrough. That said, I don't know what the last member of my team could be. This is what I've thought up for now:

Empoleon
Cherrim
Vespiquen
Porygon-Z
Houndoom

Trade is always available, and I don't care about a Pokemon having the same type as one in my list or a Pokemon being weak competitively.

Any ideas, guys?

I noticed your team is mostly special-based, so I suggest a physical-based Pokemon to balance that out. Garchomp would make a nice choice, possessing two powerful types in both Dragon and Ground, on top of great stats across the board.

I know you already have two Pokemon weak to Ice, but then again you do have a Fire-type to already cover that weakness. Gible can be caught in that secret area of Wayward Cave but be warned they can be difficult to catch.

Bronzong would be a great Pokémon to counter your 3x weakness to fighting, or a gallade.

your team seems fairly balanced (though personally i'd use Roserade over cherrim) however as said it seems to be quite special based you also have 2 electric weaknesses and 3 fighting weaknesses (though that could be fixed with roserade) as such i would reccomend Electivire

strong physical Attack which is what your team needs, can learn Ice Punch by a move tutor for the simple cost of a little underground shard hunting which covers its own ground weakness, and is straight up Immune to electric with its Motor Drive ability