Seems worse than Ghost Warden and friends, generally. This doesn't need to tap a second creature.

Does probably hit up against NWO complexity though.

Yeah, this is the kind of on-board trick that Wizards don't like at common these days. It complicates combat math too much.

(Personally I loved the Lorwyn-era complicated board states with things like Festercreep, Kithkin Daggerdare, Scarred Vinebreeder, Silvergill Douser and so on, but I can believe them that a lot of players found it too much overload. I learned to play Magic in five-player chaos games where nobody played Wraths and the board was full of bizarre effects, so my tolerances may be somewhat different to the norm :) )

Hm...now that you guys mention it, it might be a bit hard on commmon. Though I really want a temporary boost in this slot. I can make it a boring ETB effect, but eeeeh.

ETB-triggered temporary boosts are how Wizards will do it these days. They can be pretty potent - Efreet Weaponmaster is strong - or pretty basic - Kinsbaile Skirmisher is back in M15. But Wizards currently believe that repeatable effects like this, in the volumes you get when they're at common, cause attacking or blocking decisions to be too complicated for many players.

What if it was sorcery speed? "When a creature enters the battlefield under you control, tap ~. If you do, that creature gets +1/+1 until end of turn".

Aha, yes! That'd be following the pattern of Foundry Street Denizen, Ivy Lane Denizen etc, which seem to be NWO-compliant. You probably don't need to make this creature itself tap under those circumstances.

On the flip side, giving creatures +1/+1 on the turn they can't attack is rarely useful. (Yes yes, flash, haste)

"Whenever a creature enters the battlefield under your control, you may tap it. If you do, ~ gets +1/+1 until end of turn"

This should work.

The simplest would be to make this a sorcery-speed ": target creature gets +1/+1 UEOT", even though that's a bit ugly.

Alternatively make this a non-flying Griffin Protector, although that's a lot less useful if it keeps defender.

No, it will drop defender. Problem is that thee are already 2 white "make 2 tokens" cards at common, so I'm kinda iffy on it getting a boost from any ETB. If it tapped them, it'd feel a bit safer.

Much more sensible. A bit less exciting, which is always sad, but this is more suitable for common this way. You could have something similar to your original design at uncommon :)