As far as "fan" regrets, all-time I regret the spat between Jimmy Connors and RG that led to his banning in 1974, and his nose-thumbing refusal to return to the event until the 1979 edition. As far as 1974, Connors is still the only Open Era man bar Laver to go undefeated at the majors in a given calendar year (20-0 at the other 3 majors in 1974), and it would've been tremendous to learn if he could've taken the trophy in Paris and maintained his pace at the following major events.
I also think he'd have been a favorite or co-favorite in 1975 (when he straight-setted Borg in the Open semis on green clay a few months after the Swede won RG; and beat Pecci-Laver-Rosewall back-to-back-to-back to win the Volvo on red clay earlier that summer), and in 1976 (when he won big titles on red and green clay that whole summer, and then took 9 out of 10 sets from Kodes-Vilas-Borg to win the Open on clay, perhaps his greatest major title run).
As an aside, I'd give Jimmy a puncher's chance in 1977 too; not sure if any mere mortal beats Borg at the RG '78 edition.
Some of the above is influenced by how stats-conscious modern fans have become. It's important to remind that a cluster of players sitting on 7 to 11 majors (Mac, Connors, Borg, and to an extent Lendl although he was the first modern 4x majors a year guy), are as or nearly as great and accomplished, and from a certain vantage point more legendary, than the Open Era folks currently sitting between 12 and 17 slams. These guys literally redefined the sport - particularly Borg and Connors, who represented the last gasp of the pre-Open Era pro tour sensibilities - but they skipped some majors to grow the game and challenge its leadership in other ways, and have suffered as quantitative approaches to measuring success/achievement take over across all sports.
In terms of current players, I have some sympathy for Nadal's injury woes. He'd have been a real contender at RG 2004, Wimbledon 2009, the 2012 Open, Australia 2013, and the 2014 Open. When you're talking about a guy within striking distance of Federer's 17 majors, it's a shame Nadal didn't get a few more chances to inch closer to Rog.
Also, Kuerten's hip. He was making a similar leap from clay- to all-court great that Nadal made before that damn hip injury.