The game type would be... sort of a mix. Attack/Defend CTF, if you will. Blu acts as security guards and Red acts as thieves. The goal for red is to steal the flag (art) and blu to defend it.
And so, with rough ideas and a VERY rough layout, here's what Tgame and I came up with:
Area 1
Blocks jutting from walls on outside for scouts climbing to roof
Fence blocking roof until round begins so no early access
Breakable glass panels on roof to drop down
CC-contributed artwork (IE cC members draw things and send them in as textures)
Trip lasers that play sound when Red passes through them
Flag in front of vault door on pedestal
Area 2
Increased defense to make up for the small size and straightforward layout
Red team spawns where flag was located
Vault door opens when round begins
Room 1 of vault has laser grid that does damage to reds walking on it
Grid can be turned off by hitting switch on far wall with enough damage. Sniper or scout most likely candidates, depending on long range or getting across grid
Room 2 has a few sentries to aid Blu defense, sentries on walls + ceiling
Room 3 has traps (buzz saws, lava pits, etc.) to hinder Red
Room 4 has flag on pedestal
Red Objectives:
Area 1: Red spawns outside of museum. Objective is to get to the flag, grab it, and get back to the van outside the museum (near Red spawn) to capture it.
Area 2: Red spawns where Blu spawned previously. Goal is to get to the intel and run up the stairs to the roof and up to a helicopter to capture it.
Blu Objectives:
Area 1: Blu spawns surrounding flag at the end of the museum section, just before vault. Objective is to defend flag at all costs.
Area 2: Blu spawns surrounding flag at the end of the vault section. Objective is to defend flag at all costs.
Layout:

Just a reminder that this is very rough. I have a grand total of maybe 10 hours of mapmaking experience, so obviously this map has a 99.9% chance of being terrible with these ideas and layout.
Tell me what ya think.