This is an instant, so you can combine it with Phyrexian Dreadnought. Any other ideas?

To me, it seems to be an 'obvious' outlet for the garbage power of Decayed zombie tokens (which I generally don't like as the previews so far seem to cost Decayed zombie tokens at the same value as prior blocks did regular zombie tokens)

Obviously, works with Righteousness. Ball Lightning/Elemental Appeal and Feldon of the Third Path also work, as does Angelic Favor. I think Tombstone Stairwell might be the best combo, though, with anything resembling a full GY.

Hobbling Zombie is a common 2/2 deathtouch for (already passes vanilla test) that makes a decayed token when it dies.
Gisa, Glorious Resurrector is a 4/4 for 4 that reanimates all opponents' creatures that die with decayed. Lim-Dûl the Necromancer does something similar as a 4/4 but costs 7 mana and attaches a cost to it.
Jadar, Ghoulcaller of Nephalia is a 1/1 for 1b that can make a 2/2 token on each of your turns, including the turn he enters play. Aphemia, the Cacophony can do something similar but you have to jump through hoops to feed her.

I don't think they are costing the decayed tokens the same.

I'm not sure what your "Vanilla test" is but Hobbling is a little expensive for its body -- at least nowadays when Black gets Gray Ogre++ like Arrogant Outlaw on the regular (and deathtouch becomes rapidly less valuable on higher power creatures.) Whether or not it's a good thing that Ogres and Pikers no longer excite is another debate. I used Doomed Dissenter a good deal in limited, and am typically really fond of cheap deathtouchers, but it does neither job well.

Jadar, unlike Aphemia, can never build up (since his ability does not fire if you already have a token), isn't a flying piker on his own, and doesn't contribute to getting stable because the blasted decayed tokens can't block. Because of the "only one ever" aspect, Ophiomancer is the better comparison, and the degree to which Ophiomancer, even for 1 extra mana, is so much a better card is hard to overstate.

Gisa is an exception, in that she's pretty clearly very good, but she's not making 2/2 trash zombies, she's making whatever letting you cheese ETBs static abilities, and so on. I really want to abuse the hell out of her with Teferi's Veil but that's another thing. Compare Ghoulcaller's Harvest with Tombstone Stairwell. The tokens are about the same -- a Decayed zombie and a Tombspawn are basically both death triggers waiting to happen -- but for similar mana over two turns (Paying Stairwell's CU once versus flashing back Harvest) Stairwell will, in a standard duel, make about four times as many bits of disposable junk for you.

Perhaps I was a little harsh saying that they were being priced on the same level as generic 2/2 zombies, though there are some really efficient ways to conjure those up en masse, but they do seem to be over-cost as blockable shocks or death triggers. Maybe I'll change my tune after playing with them -- I certainly underestimated the reliability and therefore value of Escape in limited -- but that's what it looks like right now

The vanilla test is a basic Limited test, you check whether a creature card is worth its cost just its core features, though I am stretching the definition a little by including deathtouch in the assessment (a French vanilla test). A 2/2 deathtouch for 3 is just base line for a common: Daggerback Basilisk, Kraul Stinger, Noxious Groodion, Pitiless Gorgon. Going over that like Yuan-Ti Fang-Blade already makes the creature cost-effective.

Another common decayed token maker

Is Startle the closest we've come to a blue bear? It's an 2 mana instant that makes a 2/2 (at instant speed), and yeah it has a drawback, but it also cantrips. Seems like a pretty good card overall.

Now that one I like the itemization on.

We have had quite a few things that are very close. Departed Deckhand is probably the smallest drawback at lower rarities. And that assumes you don't count artifact bears that can go in a blue deck Watchers of the Dead

Fog Elementals don't really count as vanilla creatures for purposes of what blue gets