They have droopiest ears

Lineless version below

WindClan: Leafkit

Daughter of Featherpelt and Oatclaw, her littermates are Branchkit and Grasskit, and her older siblings are Whistlepaw, Flutterfoot, and Songleap. They focused a lot on Leafkit in Wind, so I think she will likely be a POV character, especially since Whistlepaw is her older sister. The story team also said that Whistlepaw will become more prominent in the series, so having her younger sister as a protagonist will definitely serve that purpose. Adding onto this, WindClan has never had a POV character in the main series, so this is long overdue.

ThunderClan: Stempaw or Graypaw

They are Spotfur and Stemleaf's kits and perfect protagonist material. Related to Firestar? Check. Parents were plot relevant? Check. ThunderClan? Check. They are also being given a lot of attention right now. I do not think Bristlepaw will be a protagonist, due to Bristlefrost having the name of Bristlepaw one arc earlier. (Leafkit will not have this same issue as The New Prophecy was six arcs earlier.)

These are the only candidates who have really received attention, but there are several others to consider.

RiverClan: Floatkit, Rapidkit, or Troutkit

They are the kits of Havenpelt and Sneezecloud, and younger siblings to Splashtail and Fognose. They will have to deal with their older brother's legacy once he is gone. They could also provide a window on how RiverClan is doing after this current arc.

SkyClan: Ridgekit or Duskkit

They are the kits of Blossomheart (sister of Hawkwing) and Harrybrook (son of Leafstar). Duskkit is most likely named after Duskpaw, Blossomheart's dead brother.

ShadowClan: Birchpaw

He is the son of Tigerstar and Dovewing, and younger brother to Shadowsight, Pouncestep, and Lightleap. He appeared in Sky when his brother Rowankit died. I think this is the least likely option out of all of them, as I do not see much of what Birchpaw could bring as a protagonist, besides being the son of Tigerstar and Dovewing. That is unless, some sort of huge change happens within ShadowClan; then Birchpaw could provide a view into ShadowClan.

Other possible apprentice/kit options just for the heck of it (not including anyone who will receive their warrior name/medicine cat name very soon):

SkyClan: Beepaw or Beetlepaw, kits of Nectarsong and Dewspring (who are first cousins, of course)

WindClan: Grasskit or Branchkit, littermates of Leafkit; younger siblings to Whistlepaw, Songleap, and Flutterfoot; and kits of Featherpelt and Oatclaw.

ThunderClan: Bristlepaw, daughter of Stemleaf and Spotfur, and sister to Graypaw and Stempaw; or Wafflepaw

Of course, they could always choose to focus on a young warrior, like how they did with Sunbeam this arc, but these are just my speculations.