Basically what it says on the tin.

I've played this mod a few times and I really like it, its pretty silly and very fun to mess around with - exploring random maps from the workshop and stealing everything in it is really cool, however I've realized that this gamemode is really missing something.

As far as I know, nothing happens for staying in a map for too long, meaning you have no pressure to get as many things as you can, because it boils down to just how patient you are. And with enough people and enough upgrades, it ends up feeling samey really, really quickly. The lack of pressure to steal stuff, or any punishment for sticking around too long really deflates any form of excitement you might get aside from the usual "oh look at this weird map" thing, and it becomes repetitive really quickly.

I'm unsure if this would be an unpopular opinion to have, but I think the gamemode would be significantly more engaging if there was something encouraging you to get out as soon as you can, but also enticing you to stick around for more loot. I'm not saying "add a spooky monster that KILLS you", but I think something as simple as a ticking timer activating after a while would work wonders.

Think of it like this: once you're able to steal brushes, you essentially steal reality itself (even the skyboxes aren't safe from you) - so what if, after stealing enough of a level's "integrity", you'd trigger a sort of "reality meltdown" where a ticking timer would begin as reality collapses in on itself, urging you to get out while you can. However, during this timer, props become far more valuable, enticing you to still try to stay around. If the timer ends, the level breaks completely, you lose everything and get kicked back to the bar.
This, in my opinion, would make it far more engaging for groups of people playing the mode, it'd feed into the need to buy more upgrades so you can steal more in less time (you could even add an upgrade that increases the time you have to flee) and add that currently lacking pressure while playing in a level. I'd probably make it an option however, as this would essentially remove the ability to steal an entire map (unless you're super upgraded or have tons of friends with you), but overall I think it'd be a net positive.