Hello and welcome to Heroes of the Storm,In this particular little list I wanted to list some customizations which I found from hots community,
Not About Heroes Or Skins ( a little bit of skins maybe) so if you have any ideas for other items in collection write it down and lets add it to the list !
Starting from the collection tabs third customization
Mounts
-Drakes of the wind
-Jellys
-Alliance Motorbike (cause its cooler than horde one)
-Rideable Dragoon (maybe standing or sitting on it)
-Heirloom Mount with its chauffeur !!
-Frog Mounts (size of maybe dark nexus beast)
-Infected Horses/Beasts.
-Carbot Horse (Just add it already)
-Ashes Of Al’ar (which is hard to make cause of particles I believe)
-Bronze Dragonling just for chromie 
-Hellish Beasts like Cerberus,Manticore etc.etc.
-A Huge As(x2) Raven to go with the Raven Lord Theme
Banners
-Hero Based Banners (For OW/Diablo)
-High Heavens/Hell Banners
-Infected Terran/Purifier Protoss/Primal Zerg Banners (Different Banners not Recoloured)
-Weapons As Banners (Froustmourne,Dragon Sword,Red Warpblade of Alarak,Warglaives Of Azzinoth etc.etc.)
-Banner of Scarlet Crusade,Silver Hand,Illidari,Twilight Council,Iron Horde,Scourge Banners,Troll Tribe Banners and so on For Warcraft.
-Some of the Sprays as Banners would be pretty cool too (like mr.Bigglesworth)
Sprays
-Hero Specific Sprays for Warcraft and Diablo Category
those sub-categories look like they’ve been abandoned,especially Diablo one.Some of the Portraits would be cool for those like achievement portraits of diablo or Warcraft skill Portraits. Or Well as I pointed out Hero Specific sprays like Soulstone or book of cain as spray.
Announcers Dammit!
WC
-Cho’Gall
-Samuro
-Medivh
-Ragnaros
StarCraft
-Stukov
-Raynor
-Probius
-Dehaka
Diablo
-Leoric
-Malthael
-Li-Ming
OverWatch
Still Don’t understand why Tracer is not an announcer already
and Lucio
Nexus
-Carbot Announcer (everyone just wants to hear that dude )
-THE LOST VIKINGS ANNOUNCER DAMMIT! JUST DO IT BLIZZ.
Voicelines
Seems like 5 vl is the limit but if you guys add another slot consider adding
-Boss encounter voicelines from the respected games for Villians and Well,Bosses
Emojis
-More Letter Emojis like R.I.P. or EZ (enemy team can’t see it anyways)
-1 Animated emoji for heroes ? a cry for Arthas a laugh for Lunara etc.etc.
Portraits
Brawl Portraits.
looking forward to add your ideas or the ones you came across on web to the list.