As there are 16 teams in the National League and 14 in the American, Colorado plays Pittsburgh tonight in the only non-interleague series of the weekend. I think it was originally suggested by Bob Costas, but I've long thought it would be a good idea, as long as we're going to have interleague play, to balance the leagues by moving an NL team over. (Costas suggested the Astros to the AL West, which makes sense seeing as the Astros don't have much NL history, the divisional rivalry it would create with the Rangers would be fruitful much like Cubs-Brewers, and logistically it would make for easier schedules for Oakland, Seattle, and Anaheim, who could make several one-week, two-series sweeps to the Texas teams during the season rather than having to travel from Texas to the east coast or Texas to Washington state all the time.)
The "downside" of this, as far as MLB is concerned, is there would have to be an interleague series going on every day of the season. I think the "pod" system they use now is contrived and kind of weird (and makes for more travel hassles), and there's no reason they couldn't use the last year's strength-of-schedule to determine which teams finish out of the league the way the NFL does. Overall fairness (i.e., having one six-team division and one four-team division) is more important than the minor horror of a long-shot NL wild card contender finishing its season with a DH in the lineup. Of course, this year, were the Astros in the AL West, what a ghastly division that would be. You can't please everybody I guess.