I think I would call out emblems specifically because of how broken this would be in Planechase otherwise. Then again, maybe the balance of Planechase isn't your (or anyone's?) great concern.

The card is unofficial anyways, and even if made official (or if you are playing allowing unofficial sets which includes this card), some cards can be banned or restricted in some formats if necessary.

Also, emblems are not the only objects that can occur in the command zone; so can: conspiracies (which can be face-down if they have hidden agenda, and then turned face-up later), schemes (although they will unless you have ongoing schemes there won't be many that are face-up), commanders that haven't been played yet, vanguards, etc.

This isn't meant to be limited to only emblems; it is supposed to work in conspiracy, commander, etc also.

Honestly, it doesn't seem like this will be a huge problem card no matter the environment. Sure, it might be a fatty; but cost it as such and you're done. It doesn't have any game breaking abilities, and as long as you have to wait a few turns to cast it, it can be chump blocked.

It doesn't look broken in Planechase, since it doesn't see face down objects, and there's generally only one Plane face up (plus maybe briefly the occasional Phenomenon).

Costing it at, say, 3 would likely be fine. Tarmogoyf was the outlier among Lhurgoyfs at 2 mana; the rest are mostly 3-4.

Now there is also the junkyard; Attractions in the junkyard will also be counted.