Deathtouch and unblockable aren't exactly synergistic. I think it should be one or the other, and that whatever you choose, this creature should have that ability. If you have it grant unblockability, there's very little reason for it to have flash. If you do deathtouch, giving this flash makes it a pretty good combat trick for blocking.
On another note, I don't think in the casting cost works for an ETBs ability on a creature. can modify how a creature enters the battlefield, in terms of it entering with +1/+1 counters, but I think in order for to apply to the ability, it has to be worded like this:
"When ~ ETBs, you may pay . If you do, X target creatures (are unblockable this turn/gain deathtouch until end of turn)."

I think in the mana cost can work: Maga, Traitor to Mortals. This could be templated like Squealing Devil equally, but I think this way can work. (EDIT: Huh, no, it seems I'm misremembering. I suppose the Squealing Devil template would be best, but I don't think it matters very much. I think raggarn is probably more interested in design critique than templating critique.)

I agree that deathtouch and unblockable rather work against each other.

Eh; one's using it as an attack creature; one is using it as a defence creature. 4/4 is perfectly usable as either.

Not everything needs to be hyper-focussed.

Yeah well the reason it's DT and unblockable is to make this feel more blue trickyish. So that you will have two choices instead of one like the other creatures. You may choose to try and go for lethal with unblockable on your turn. Or wait and see if you could use it as a combat trick. This way i think the design of it will make the games more fun instead of a card with just one side to it!