Before he's done, Messi could very easily have basically every goalscoring record there is:
LA LIGA:
Most goals scored in a season: Already has the record, with 50 last season
Most goals scored in a calendar year: Already has the record, with 43 in 2012 already
Most goals scored in a career: He needs 70 more La Liga goals to break this. He will surely get this record within the next few years.
CHAMPIONS LEAGUE:
Most goals scored in a season: Already has the record, with 14 last season
Most goals scored in a career: He needs 19 more goals to break the record. He could easily break this next year. Most likely, it'll take a year or two longer than that, but he will definitely get it.
OVERALL:
Most goals scored in a season: Already has the record with 73 (82 w/ NT added) last season
Most goals scored in a calendar year: He could easily finish this calendar year with 85+ goals. It'll be tough but it's definitely doable.
Most goals scored in a career: Pele has his silly 1000+ record, but realistically, he actually got 757 goals. Messi needs 457 more goals to break this. He PROBABLY won't break this, but it really is possible. Let's say he gets 50 more goals this season and then averages 60 goals a season for the next 4 seasons. That would put him at 591 goals at age 29. He would need 167 goals after age 29 to break Pele's record. That would be real difficult, but not impossible. Gerd Muller got about that many, for instance. It may seem absurd to assume a player will get 60 goals a season for 4 seasons, but Messi has averaged that much the last three seasons and is actually getting better each year (as you'd expect for a player of his age). If he kept up the goalscoring of last year (and the beginning of this year) for the next 4 years, he would be quite likely to end his career ahead of Pele. I expect him to fall short of this, but I think he will end up with somewhere between 600 and 700 goals.
ARGENTINA:
Most goals in a year: Messi already has that record, with 12 this season, and another friendly to add to that.
Most goals in a career: He needs 26 goals to break that record. If he continues his current Argentina form, that'll be easy. Even if he doesn't, there's a very good chance he will end up with this.