Ivanovic, Vaidisova, and Jankovic are all already better clay courters then she is, and they are in her age range(Vaidisova younger, Ivanovic same, Jankovic older)and have not matured their games to as much of their potential as she has. If any of those had played her at this years French Open she would have lost. I gaurantee you that much. So I dont see any future French Open titles for her. Henin will be tough to beat at the French, and a major force in the womens game for another several years anyway. Serena too if she wants to be. Kuznetsova is generally a better clay court player then Sharapova, so if she stays in the top group of women another several years she is another one over Maria in French Open odds. She too would have beaten Maria had she played her here. Dementieva and Petrova are better clay courters if they get their acts back together, and could have several more years as contenders should they get it together again, although that is a big if with them. Jankovic, Ivanovic, and Vaidisova will be around a number more years, and all will have better chances then Maria at the French Open on a regular basis. Then after that there are more players we dont even know anything about yet. A next generation will come, then a next, then probably a next, each including several players who are probably better clay courters then Sharapova. So no a French Open title for her in the future would be a miracle.
I dont even think she has that many more slams outside the French. Vaidisova, Ivanovic, and Jankovic can be better players then her on every surface, and in a year or so will be IMO. Kuznetsova, Petrova, and Dementieva could be if they fulfilled their potential as players but sadly that is looking less likely. Henin is unbeatable for Maria on clay, beatable by Maria on hard courts or grass, but Henin is still favorite even on hard courts or grass. Serena can be beaten by Maria on certain days, but a motivated and confident Serena is much too much for Maria. Venus is done as a contender at the French I think, but not neccessarily from a contender at Wimbledon and the U.S Open if she can regain confidence and motivation. Mauresmo, Hingis, and Peer might be threats as well, although I doubt it. Then like clay the new players to emerge, although they will have a harder time being better then Maria on non-clay surfaces as clay I concede.