While this idea sounds interesting, I don’t think that it would be consistent enough with the other modes
Modes are meant to make the game chaotic and/or interesting, and I think that for this mode to fit well in the game, it should be on a similar level to other modes, like Balanced or Rising Lava for example. Also, for consistency, I personally don’t think that the time limit should be changed unless time is directly related to the mode
A mode where you can’t place blocks means limited actions. While you can jump down to the ground level in some maps (like the map with slime blocks at the bottom) and fight other players there, the gameplay aside from that would mostly involve waiting on your pillar to get a usable item, which wouldn’t be the most exciting situation.
Fortunately, spawn eggs might come in handy in this mode as you might be able to spawn hostile mobs on nearby pillars, but even with that feature the gameplay wouldn’t be the most exciting.
In addition, there’s not a lot of strategy to throwing a projectile at a player that’s standing on a singular block and doesn’t anywhere to move to. I feel like the fighting that would take place on the pillars would mostly be hitting easy targets
Since you wouldn’t be able to place blocks, would blocks also not appear in your inventory? If that’s the case, then you’d be more likely to get certain items (like snowballs or spawn eggs) that you can use against other players, making the round more likely to fit well within 5 minutes
I feel like having blocks in your inventory that you can’t place would be kind of redundant. While blocks like wooden planks or cobblestone can be used for crafting, a crafting table would have to be placed down for most recipes
An idea is to make it so that there’s a rare or uncommon chance for any player to get a block (that can be placed). This ensures that players can’t get to each other as fast as in other rounds while also allowing blocks to be used