Given community day talk, I was curious about looking into likely candidates as the year goes on. Of course, we have some parameters we can use to narrow the list down. I've put together a list of trends/major requirements we can observe (and that should already be obvious) -
1. Every other month is a starter in Pokedex order (barring December's roundup).
2. Besides Eevee, for unique reasons, every community day Pokemon has been a 3-stage evolution.
3. Niantic has been utterly reluctant to introduce later-introduced pre-evos (Igglybuff, Azurill, etc.) in the wild, which might negate their appearance for community days.
4. Let's assume, for our collective sanity, that Niantic won't do a Beedrill community day and will disqualify very common and utterly meta-irrelevant species.
With that in mind, here is a list of 3-stage evo Pokemon as of Generation 4, as I am operating under the assumption that Generation 5 won't be around until late this year and thus not eligible for 2019 community days, with my above guidelines factored in. Furthermore, I'll lop off previously featured Pokemon by listing them first. I've taken the liberty of pruning down obvious above-point 4 Pokemon; feel free to disagree.
Previously featured community days in order -
Raichu (Jan 2018)
Dragonite (Feb 2018)
Venusaur (Mar 2018)
Ampharos (Apr 2018)
Charizard (May 2018)
Tyranitar (June 2018)
Blastoise (July 2018)
Eevee (Aug 2018)
Meganium (Sept 2018)
Metagross (Oct 2018)
Typhlosion (Nov 2018)
Feraligatr (Jan 2019)
Now, assuming starters remain on the normal schedule, let's add -
Sceptile (Mar 2019)
Blaziken (May 2019)
Swampert(July 2019)
Torterra (Sept 2019)
Infernape (Nov 2019)
Empoleon (Jan 2020)
Which leaves us with February, April, June, August, and October 2019 to fill out, assuming that this December will be another roundup. So, choose from the list -
Vileplume / Bellossom
Poliwrath / Politoed
Alakazam
Machamp
Victreebel
Golem
Gengar (was previously in a special raid day)
Crobat
Ludicolo
Shiftry
Gardevoir / Gallade
Slaking
Aggron
Flygon
Walrein
Salamence
Luxray
Roserade
Garchomp
Weavile
Magnezone
Rhyperior
Electivire
Magmortar
Togekiss
Mamoswine
Porygon-Z
Dusknoir
My personal assumptions -
Gardevoir / Gallade - Gives them a twofer and perhaps an excuse to introduce the next round of Gen 4 evos
Salamence - Obvious reasons
Garchomp - I think this one could be used to introduce a new Pokemon by way of community day, which might be very handy in Gen 5 if they keep the slow trickle and keeping in mind that generations plethora of 3-stage evos (20 of them!).