Spoiler for Season 19
My theory is that the next season will be about Any and Nathan and Nathan leaving for Salt Spring by season end and the final being Any and Nathan engagement , as they have already closed Georgie's Arc
In non Amy romance next season will concentrate on the love triangle between Katie (Amy) ,Dex(ty) and River (Ashley) brackets being what they clearly mirroring on.
As Jack said in last Episode Katie will be moving to college in year or two so the series will end with Amy, Lyndy and Nathan moving to the island, Katie goes to college maybe Vancouver, Jack and Lou will stay back at Heartland, and in the end you will see Any riding Spartan one last time across Heartland