As a non-plat user myself, i have seen that after some significant work in merching and farming, i have no good pets available to me without paying out a huge chunck of money.
With almost all the myth/ arcane pets from locks and direct plat buy, i have a different idea to reward the hardworkers

Why not have a huge quest ( and by that i do mean huge questline) to obtain an untradeable mythic pet.
Just an example i thought over on my ride home.

A new npc in tindirin bearing a message
" Thank you hero for your help in saving arlor from those savage creatures yet again. Following your conquests, I have found a dragonstone. Yet it is dead, a fossil.
As an expert in draconic species i would condemn it dead yet no one can deny the sudden heat it emanates when you approach closer.
Do you think this could mean something? Hax/Hox ( those seer guys) tell me part of the secret lies in the arena of heroes."

Then every arena run drops charred scales, we will need like 80-100.
Back to NPC
" this is nothing like i have seen before, the scales on this egg were stone, yet they seem to meld into the egg , it almost seems alive now"
to seer hox/hax
"my friend has told me about these queer occurences. the planar regions have always held strong magic whose origin is known to none, mayhaps they came from dragons of old. Keep investigation
New quest=> defeat ashral heart and elite inhan for letters
back to hox/hax
" this language is queer. It tells of a legend old, when dragons roamed the dusty lands of arlor. For this you need to talk to the oldest creature of this land; over mortal enemy Taurus the Hammer. Beware he measures strength with courage.
Defeat the guy 'x' number of times to earn his favour. He will pass on a shard.
back to NPC
"this shard is like no other. What a fool i have been, this is a claw ; a victory loot of taurus. Yet he has never come near arlor, we will need more signs.
quest=> defeat A,B,C,D elite bosses for more remanants.
NPC
"I had feared the worst. The alchemist asks for you, and not kindly. Be careful"
Alchemist
quest-you will need x,y,z "looted" pets before he talks to you
" I see you are capable. I shall craft you a companion like no other, and breathe life into it. Yet i need one final thing from you. No it is not another rare item, just the usual barter, gold" (200k i think)
crafted...voila! new mythic pet ( probably recoloured pet)

I do not know why i put so much thought into an example, i think i have to much free time