bringing jayce/viktor back in canon would be sooo incredibly difficult.
first of all, most postcanon fic authors struggle to give jayce a compelling motivation/arc bc hes VERY tightly written. he outright says that his goal in their final scene has shifted from bringing magic to the world to getting viktor back. and by staying with him, he gets viktor back and faces the consequences of his actions throughout the show. everything is wrapped up pretty neatly for him.
(viktor isn't AS difficult. It's pretty easy to create a new story arc for him out of the consequences of the glorious evolution and transforming his outlook and actions based on his realization at the end of the show. most fics ive read go this route, with viktor dealing with a lot of guilt and reflection and finding a new purpose and sense of self.)
So the first thing they'd have to do is come up with an arc for jayce. but also, said arc still has to heavily involve viktor. the nature of their story basically requires they remain together for the most part. jayce getting viktor back at the end of arcane is the catharsis for his story. if they separate (even if it's realistic or good for them or whatever else) it makes that conclusion lose a LOT of its impact and meaning. likewise, retreading on that conflict also dulls the impact, so even if their new goal was to reunite after being separated youd run a HUGE risk of repeating arcane's plot points and themes, making them feel stale.
(I've seen some people theorize that jayce may get separated from viktor and join mel in noxus. unfortunately this is even worse imo. youre risking either retreading old ground TWICE or a mix of retreading plus making the ending lackluster and making jayce act ooc)
so we need a new arc for jayce AND we can't separate him and viktor in order to accomplish that. thats already hard. but that leaves a new question, why bring them back at all? what do they have to add to the future storylines in runeterra and how do potential new arcs for them support the themes of these stories? and why bring THEM back instead of introducing other characters from league lore that haven't gotten a chance to be explored yet?
these new spin off shows are going to be focusing on different regions of runeterra that already have so many characters and stories to pull from. and yes they'll definitely tie the events of arcane into their stories and we will likely see some of the characters again. but bringing back jayce and viktor isnt as simple as having a caitvi cameo or having jinx show up in bilgewater or something. it would require a lot of set up and precedent and theyd need to contribute more to the story to justify that.
it just doesn't seem very likely unfortunately. and id be worried for the story quality if they were to go this route. i still think the best choice is for them to haunt the narrative going forward, either literally or metaphorically. characters could mention them and what happened to them as a cautionary tale or if they still exist within the arcane maybe theyll be ghostly figures that literally haunt swain if he tries to use the crystal that his raven stole. theres a LOT more potential and safety in that writing choice imo.