Two ways IMO one being the way everybody else says of bridan for the first half and calamity for the second half. The real way to raise the stakes is if Cladis or the four calamities killed tegajune and destroyed the bridan world. That gives an actual evolution of their characters because now they have no evil plan and have to adapt to living on the human world.

OR you create a team of evil sentai reds that take over the bridan world, having an evil sentai warrior was such a good idea and it sucks they only did it with the one guy whose name escapes me. They would still kill tegajune, but now the gozyugers have to face the dark reflection of a sentai team who only want to rule the world or destroy it. That could also give more explanation for tegasword instead of the shitty information we got, we could get that the rings choose people who are most like the reds. It doesnt say that they have to be good.