6th Gen Pokemon X team building help!

Ok so I'm planming out my team for Pokemon X bevause I recently decided to re-play it.

This is my team so far:
Delphox (Fire/Psychic)
Vivillon (Bug/Flying)
Aurorus (Ice/Rock)
Sylveon (Fairy)
Goodra (Dragon)

My issue is I can't decide which Pokémon my last part member is. Ideally, I want a Pokemon with high speed and attack since my team is heavily specially defensive and all except for goodra are special attackers.

For a fast physical attacker I was thinking:
Mienshao (Fighting) or Flygon (Ground/Dragon)

Other options I thought of (that don't fit the fast physical attacker ideal):
Venasaur (Mega): (Grass/Poison)
Lapras (Water/Ice)
Furfrou (Normal) - great ability

I honestly can't decide so I would appreciate some help and let me know if u have any other suggestions! Thank you!

Hawlucha is pretty fast and strong

I think Mienshao would be a great choice. It is both relatively fast and strong plus it has rather good type coverage. :)

One of the things that I most value when building a team - besides personal preference for the designs of some species - is type diversity. The optimal configuration is twelve distinct types being represented; the minimum for me is around six before I'm satisfied.

Looking at your current team and your options, you might want to go with Mienshao. It fits the kind of speedy physical role you're looking to fill, and its type isn't one that you've used yet. Flygon would be a second Dragon-type, and would give your team a 4x Ice-type weakness on top of already having Goodra and Vivillon, which are weak to Ice as well. With Mienshao, you have nine types represented, without any overlap between them - that's a diverse and effective combination, and Fighting helps with your coverage against opponents.

While some of the others would introduce even more type diversity to your team, you're right to want a speedy physical attacker - that could prove very useful for your team, especially when dealing with some of the late-game gyms and the League.

In some ways, you kind of have to go with Lapras since you need something to Surf/Waterfall on, and nothing on your team can use those moves. Kalos isn't overflowing with water like Hoenn is, but Surf+Waterfall is still required to progress through the game.

Honestly, I've already decided on Mienshao. I think I'll make Lapras sort of a 7th team member if you will, and I'll primarily switch it out with another Pokemon in areas where surf/waterfall is needed. But thank you for bringing this to my attention, I totally disregarded it.

Here is an ok team-
1.Greninja
2.Talonflame
3.Trevenant.If you don't have 2 ds's use the gts to find a phantump.
4.Dragalge(Pokemon Y Exclusive), or you can go for a Tyrantrum for more coverage.
5.Lucario(Gift Pokemon)
6.Aurorus(Or if you picked Tyrunt, Mamoswine would be good)
Hope this helps!