Major League Soccer story lines: Five things to watch during the 2018 season

Jim Reineking | USA TODAY

Show Caption Hide Caption MLS makes push to sign young South American players As MLS continues to develop as a league, it has made a concerted effort to sign some of the world's best prospects.

As Major League Soccer prepares to kick off its 23rd season, some key story lines to follow:

Toronto FC's encore

Last season, Toronto FC became the first MLS team to win a domestic treble (domestic cup, Supporters' Shield and MLS Cup). In accomplishing that, TFC has earned recognition as the best team in league history. The core of this team is intact, so expect more big things. Jozy Altidore, Michael Bradley and Sebastian Giovinco will be tasked with leading TFC on its quest to repeat as champion and match the back-to-back championships of the 2011-12 Los Angeles Galaxy.

South American imports

The idea of MLS teams acquiring young, talented South American players isn't new. It's just that these players are being acquired in a more frequent, and expensive, rate. Atlanta's MLS-record transfer fee paid to acquire Ezequiel Barco was viewed as a significant moment for the 23-year-old league. Atlanta will have to wait for Barco's much-anticipated debut, as its new 18-year-old midfielder will miss the first four to six weeks of the season with an injury.

However, Barco isn't the only promising, young South American import coming to MLS in 2018. Eduard Atuesta (LAFC), Josue Colman (Orlando City SC), Jesus Medina (New York City FC), Diego Rossi (LAFC), Jeisson Vargas (Montreal Impact) and Milton Valenzuela (Columbus Crew SC) also were brought in as "designated players" to be immediate difference-makers for their teams.

MLS 2018 predictions: Can anybody knock off Toronto FC?

MLS preseason power rankings: Toronto FC remains MLS Cup favorite

More: Expansion LAFC sells out of season tickets at new stadium

Los Angeles Football Club

The long-anticipated debut of LAFC is here. Will the team be worthy of the hype train that's been rolling for nearly three years? Expectations are sky high for this team, which will play in a gorgeous new stadium located adjacent to downtown L.A. In the long term, there's little doubt the ambitious franchise has the capacity to reach those incredible expectations.

In the short term, however, LAFC will be hard-pressed to match the amazing inaugural season of last year's expansion darling, Atlanta United FC. Like Atlanta, LAFC hired a big-name coach (former U.S. national team coach Bob Bradley) and acquired promising South American players (Atuesta and Rossi). LAFC also signed Mexican national team star Carlos Vela. Anything short of a playoff berth this season might be considered a disappointment for L.A.

Young Americans

The U.S. men's national team's shocking failure to qualify for the 2018 World Cup has forced the program to dramatically shift gears and look (way) ahead to the 2022 World Cup. A good portion of that next generation of potential USMNT players is already developing in MLS. At the U-17 and U-20 World Cups last year, the USA was one of just two countries to advance to the quarterfinals of each tournament (England, which won both, was the other).

One of the key players on that U.S. U-20 team was Tyler Adams, who was a revelation for the New York Red Bulls last season. He's only 19, and a strong case could be made that he would have been on this year's World Cup team had it qualified. Also on that U-20 team with Adams were Danny Acosta, Justen Glad and Brooks Lennon of Real Salt Lake and Eryk Williamson of the Portland Timbers. The U-17 squad featured Ayo Akinola of Toronto FC; Andrew Carleton and Chris Goslin of Atlanta United FC; Chris Durkin of D.C.; Jaylin Lindsey of Sporting Kansas City; and James Sands of NYCFC.

Expansion

MLS continues on its crusade toward 28 teams. Nashville and Miami (at long last) recently were announced as the league's 24th and 25th franchises. Soon, the league will announce team No. 26, with Cincinnati, Detroit and Sacramento in contention.

Follow USA TODAY Sports' Jim Reineking on Twitter @jimreineking.