The MLS is a very unique league, and it will be interesting to see how City (if they do actually get a team) integrate into the league. Mexican club Chivas de Guadalejara tried a similar expansion into the MLS, and was/is an utter failure. I live in Seattle, so was able to first-hand experience a very successful start-up. I think the Seattle Sounders earned the title of one of the most successful (expectation-wise) expansion teams in US sports history.
My advice to City would be this:
1) Invest heavily in marketing and building the brand. Seattle sent out alot of consumer surveys years before announcing a team, held a public vote on a possible 4 names of the team and a vote on the crest symbol. Seattle is also the only team with a 4-year vote of confidence by the public for the general manager. I think the visionary behind the marketing strategy has since moved on from the club, but if I was City, I would look the guy up.
2) Make sure the stadium is worth attending. Since we have no history in the game, Americans want to attend a game in a nice stadium with good overall atmosphere that makes you feel special that you're there. Sounders play in an NFL stadium that is only allowed 1/2 capacity (35,000), but it is a great venue designed with a view of the downtown skyline from almost every seat.
3) Make sure you understand the league before assembling a squad. Many clubs think you can bring in a successful european coach, and some european players, and think you will do well in the MLS. Those clubs do very poorly. The MLS is a physical, non-technical, league with alot of travel. The MLS has a salary cap that is supposed to protect the league from becoming too lop-sided (It's currently too low in my opinion). There are a couple "designated player" spots that don't count against the salary cap that are there to bring in high-profile name players to attract viewers. Due to the salary cap there isn't much depth in teams, so usually teams with the most balanced squads do the best. I would find an experienced and successful MLS coach, bring in a good core of MLS veterans, and get a winning first season under your belt.
Do all this and New York-City will have a good base to work from. The MLS is actually now becoming a good place to try out emerging South American talent, and bring up home-grown academy players. Whether any players become quality enough to join Manchester City down the road, we'll just have to wait and see. I wouldn't recommend using New York as a place to loan out young players, but it could be a good spot for aging players to impart their experience and professionalism.
