It's a 2/2 First Striker so it eats face down creatures. If they let it through they could lose a powerful morph trigger effect. What to do? Given the ability should this guy be rare ?

Once the set is filled out, see how many really good morph triggers you have then come back and re-evaluate the rarity.

Except that this isn't a proper Set. It is for my ideas that don't fit into one of my actual sets - Aerikal & Dark Omens.

Well, if we compare it to Magic as a whole, then, it's weak, since there are so few Morph creatures in existence.

If this was to be dropped into a set with a high amount of Morph like Onlsaught block, (not Timespiral block with smattering of Morph) it still wouldn't be that powerful. Sure, it will turn up the occasional Voidmage Apprentice, wrecking the thing, and spoil any morph surprises, but most of the time you'll just end up accelerating your opponent's deck. If I had a face down Hystrodon, and you turned it face up, I might be a little miffed that you won't miss-block it now, but I'll be happy that you saved me a whole turn worth of demorphing the thing.

If you want the card to be always good, I'd suggest "look at target face down card your opponent controls. You may turn that card face up." Printed like this, with all upside, more players would be attracted to playing the card, even if this forced you to increase the mana cost to 3W or make it a 2/1. Personally, I'd keep the mana cost to P/T ratio, though. If it isn't going into a real set, why not let it be a little spicy?

Agreed with the "all upside" comment. I think this card begs to useful at all times. As currently worded, I would only use this on my own morph creatures to accelerate them to their better versions (Akroma, Angel of Fury, anyone?)

Good points from both, I made the change.