It would be nice to have a costume toggle feature for the Colosseum to turn player costumes on/off. Maybe have the setting be: off [ALL/enemy] and on [Myself/everyone/teammates/enemy]. Why collect cosmetics if you can't show them off everywhere? Part of the fun of this game is looking cool anyway.
Costumes In PvP
The meta is so ubiquitous it doesn't matter. The most important thing to note on a person isn't their armor, its the weapon choice.
If you can hide what gear you are wearing the enemy won't know what playstyle to expect..... if you disguise as a Skolver your enemies will expect you to main swords but if this disguise is over a BK set you can pull out Max damage guns in the middle of the fight.
When disguised you can trick enemies into using less effective weapons: using Chaos while disguised as a BKC/Skolver Mix will prompt enemies to attack you with Elemental weapons which your Chaos resists.
http://forums.spiralknights.com/en/node/105548 <- this suggestion would work out much better
I don't see this as a problem because lockdown is already kind of compromised in exactly the way you're asking for- armor doesn't resist anything very well to begin with (only a little) and people are free to use trinkets + unique variants to completely change their stats as it is. Very little important information is handed to you purely based on their armor choice- if I see a skolver, I know that they're resistant to my shivermist buster.. but I have no idea if they can withstand my other haze bombs. For all I know they could be set up to walk through and resist everything I have on me. This misinformation of trinkets/armor type/"weaknesses" not working etc. is just as harmful as simply not giving out information to begin with lol.
Armours like Ancient Plate and Chaos aren't the same thing, anyone who plays LD would tell you the same. If you wanna look good just accessorize your PvP gear. Even if the Skolver can walk over other bombs, that at least tells you to not bring a Shivermist Buster in the first place.
For exactly the same reasons Holy-Nightmare gave.
It introduces more strategy and mind games into the meta, and the system is fair since all players can take advantage of it.
Why would you not want to know what gear people are actually using? You can have it as a personal option to toggle on and off, but you're not really helping yourself.