I think the Sinnoh throwback can go both ways, really. Gen IV is really light on native Pokémon, with a large chunk of its regional dex taken up by cross-gen evolutions and legendaries. Let's do a quick count:
- 29 native evolution lines (Pokémon that only evolve from or into other Gen IV Pokémon, if they do at all)
- 27 cross-generation evolution lines (7 pre-evolutions, 21 evolutions - the Budew line has both)
- 15 legendary lines (counting Rotom, but only once. For Pokémon Go purposes, where every form is counted separately, there are 22 legendaries).
If Niantic wants to restrict the event to Sinnoh natives only, I think the event could be somewhat boring. Let's break down the 29 native lines into easily discernible Pokémon Go categories:
- 3 starter lines - these will be somewhat exotic, at least
- The Starly line - common as dirt
- The Gible line - so rare that it might as well be disregarded entirely
- The Shinx line - Gen IV's only three-stage line that doesn't fit into the three categories above
- 2 fossil lines - usually Egg exclusive, gotta keep folks buying Incubators after all
- The Burmy line - seven creatures across three Dex slots
- 3 single-stage evolution families - Pachirisu (regional), Spiritomb (event exclusive), and Carnivine (regional)
- 17 regular two-stage evolution families - Bidoof, Kricketot, Combee, Buizel, Cherubi, Shellos, Drifloon, Buneary, Glameow, Stunky, Bronzor, Riolu, Hippopotas, Skorupi, Croagunk, Finneon, and Snover.
Of all these, really only the Gible and Riolu lines can be considered rare in Go under normal circumstances, and both are typically Egg exclusive to sell Incubators. As are the fossils. Shinx is a consolation price in the lootbox lottery of 10k Eggs and one-star Raids. We can also disregard entirely the three single-stage lines. That leaves 22 evolution lines for the spawn tables and research tasks. The starters, Buizel, Bronzor, Glameow, Hippopotas, Skorupi, and maybe Finneon can, at a stretch, be regarded as uncommon, but even they aren't among the most exciting of catches out there. Personally, I wouldn't be too excited to come across any of the others, as I feel like I do all the time. I mean, how many Croagunk do you have to see before you consider there to be enough Croagunk in your life? Hardly more than ten, which gives you enough Candy to get Toxicroak. After that, they're a bit of a nuisance.
I'm personally really fond of three-stage evolution lines, but native Gen IV doesn't have many to offer. There's the starters (somewhat common) and the Starly line (extremely common), but the only others are the awfully rare and egg exclusive Gible, which I haven't even got a faint hope of seeing during this Event since Niantic gotta sell their lootboxes, and Shinx, which seems to be in every other 10k Egg I hatch. I've got plenty of decently-IV'd Shinx already, and enough Candy to last me a while, to the point that finding
another Shinx is somewhat annoying, since it means I didn't get any of the rarer 10k 'mons after I just spent an Incubator. The ubiquity of Shinx really highlights the lootbox aspects of Eggs. It's a consolation prize.
In conclusion: native only Sinnoh - not much to be excited for.
However, if cross-gen evolutions are included, we suddenly have a ton more powerful and exciting Pokémon to pick between. I wouldn't really be excited for the option to catch a high-IV Floatzel (I mean, even with high IVs, it's still a Floatzel), but the prospect of getting a high-IV Electivire, Yanmega, Tangrowth, or Weavile, that's something else. There's something exotic about the cross-generation evolution lines that makes even common spawns like Misdreavus or Magnemite feel exciting. If the Sinnoh event includes cross-generation evolution lines, we could have twice as much variety, and easily ten times as many three-stage evolutions that aren't starters or Starly.
So yeah, the Sinnoh event really can swing both ways: The mediocrity of native-only Sinnoh, where the most exciting thing you can hope to find is a Bronzor, or a full-fledged cross-generation bonanza, where you'll have to prioritize your use of Sinnoh Stones. Knowing Niantic, though, they'll probably go for the former.