PEORIA, Ariz. -- It’s baseball season.

The Padres begin their Cactus League slate on Thursday, when they host the Dodgers at Peoria Stadium. They'll send Joe Musgrove to the mound for the opener.

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As ever, the big names are worth watching. But the Cactus League is about more than that. Musgrove is in the season-opening rotation, regardless of his spring performance. Whether Fernando Tatis Jr. hits .600 or .200 this spring, he’s starting in right field on Opening Day.

The most interesting spring storylines are subtler. Here’s what you should really be watching for as Cactus League play gets underway:

1. Merrill's big test
No general manager has been as aggressive as A.J. Preller when it comes to promoting top prospects out of Spring Training. (Tatis and CJ Abrams come to mind.) With vacancies in the outfield, a hugely intriguing spring awaits Jackson Merrill, the organization’s No. 2 prospect in the MLB Pipeline rankings and No. 12 overall.

There's opportunity available for Merrill. A lifelong shortstop, he wouldn't be playing the outfield if the Padres didn't envision a big league opportunity for him there. But -- like Abrams and Tatis before him -- Merrill will have to earn his roster spot with his play on the field.

"Whatever position they want me to play, I'm ready to come with the stick and be a productive bat in the lineup,” Merrill said. “I've done everything I possibly can this offseason to prepare. I'm ready to go."

2. Rotation buzz
Maybe the Padres add another starter, leaving one spot available. Maybe there are two rotation jobs to be won. Either way, there's an intriguing race -- with some electric arms -- developing at the back of the San Diego rotation.

Jhony Brito and Randy Vásquez have drawn rave reviews for their work this spring. Arrivals in the Juan Soto trade, they're the two early favorites to win back-end rotation places. But keep an eye on Jairo Iriarte, the team's No. 7 prospect. The 22-year-old right-hander made huge strides last season and has "big league rotation stuff," per a team evaluator.

Pedro Avila, Matt Waldron and Jay Groome also join the competition for places at the back end. As for Robby Snelling and Drew Thorpe -- the team's No. 3 and No. 6 prospects, respectively -- they’re longshots for the Opening Day roster. But with an impressive spring, they would force their way into the picture, perhaps setting up a midseason callup.

3. Roster battles galore
If there are 13 position-player spots on the roster, realistically only eight can be written in ink right now. That leaves five places available -- a stark contrast to the past few seasons when the Opening Day roster was mostly set, save for a spot or two at the end of the bench.

“It’s the most open camp [since] I’ve been here,” said manager Mike Shildt, who spent the previous two seasons with the Padres as an adviser. “I think it’s great. It’s Spring Training, and guys are going to get after it. Guys are going to get opportunities. You don’t want to over-evaluate in Spring Training. But the reality is we’re going to make decisions based on what it looks like.

“We’ve got open spots in a lot of different areas on our club. We’ll see who can grab them.”

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In particular, there are four intriguing left-handed-hitting prospects in the mix -- on a roster that’s seriously deficient in lefty bats. Merrill is joined by Graham Pauley, Jakob Marsee and Nathan Martorella. Although Marsee is the only true outfielder of the bunch, all four will compete for roster spots and have some measure of outfield experience. That group is complemented by non-roster invitees like Cal Mitchell and Bryce Johnson (a switch-hitter).

It’s still likely the Padres add a couple bats to the mix before Opening Day. But there will be opportunity available regardless.

4. The Ethan Salas show
At age 16 last spring, Ethan Salas appeared in the final four innings of a Cactus League game. At age 17? The Padres’ top prospect is part of the spring roster, a full participant in his first big league camp, sharing a clubhouse with players twice his age.

Salas will not be making the Opening Day roster. (When the team departs for Korea, he’s slated to headline the Padres’ Spring Breakout game against the Mariners, pitting the organizations’ best prospects.) Still, the Padres have been aggressive in challenging Salas, and this spring is no different.

Salas is destined to catch at the back ends of Cactus League games. But he’ll still be catching big league arms -- and facing big league arms at the plate.

“Pretty excited,” Salas said. “I’m just ready to go out there and play.”