I'm all for a Proxian being the new Mars Adept to replace Eoleo in whom won't return because his story along with Himi's story is over, he needs to return to Champa and take care of his grandmother.
Felix doesn't need a descendant, Matthew is already his nephew, I'd rather keep Matthew the leader I'm kind of tired of the leader changing in every title, just keep one, I like Matthew because he reminds me of Isaac and the fact that he's Isaac's son and Felix's nephew works out perfectly.
Tyrell is just someone we have to put up with as one of Mars adepts, but for the second one I don't want another Eoleo, nor do I want a clone of Jenna in whom already has Matthew as a descendant which is good enough, I want to see a Proxian as our new Mars adept.
Think about how interesting the Proxian woud be.
He/she could have moves like
Rising Dragon: Agatio's move from Lost Age could be used as a Proxian exclusive
Fume series: since the Proxian would take Eoleo'splace, then the fume series would still be usable psynergy
Flame Dragon: like Sveta, the Proxian can transform into a Flame Dragon to increase the power of his/her stats
Pyroclasm: Can't have a Proxian moveset without good ole Saturos's favorite move
Super Nova/Mad Blast: Karst and Menadri used these moves, either one would work, maybe Mad Blast since Tyrell has Supernova.
Also the weapons a Proxian would use are
Scythe: exclusive to Proxians and makes a great exclusive new weapon to use
Long Sword: Like Matthew, he/she can be able to use long swords, however unlike Matthew and the others, the Proxian holds it like it's a light blade, much like Saturos did.
Light Blade: Can also use light blades
Fist: Like what Agatio probably used, there would bea second exclusive weapon for the Proxian, fists.