The Da Capo Universe:

Da Capo's first anime season covers the original Da Capo game.  D.C.S.S., the second season, borrows from Da Capo Plus Situation/Communication while mainly telling its own wonderful story.  Da Capo II's first season covers Minatsu's arc from Da Capo II.  D.C. II S.S. covers Anzu, Yume, and Otome's arcs from the D.C. II game.  Da Capo III's anime covers at best the common route of the D.C. III game and then ends abruptly, solving nothing.  Hopefully a new season of Da Capo III will be made which will go towards solving all the questions the first season raised, but since it had low sales, even that is unlikely.  Da Capo If tries to represent Kotori and Jun'ichi's romance, but it doesn't follow the actual Kotori route from the original Da Capo nor does it cover the entire D.C. If visual novel, so it's pretty much a waste of time.  Likewise, the two episodes of Time Paladin Sakura don't in the least cover the two visual novel games of Time Paladin Sakura, and serve only as teaser trailers.

Even though Da Capo is a very long anime, it hasn't even come close to fully realizing the franchise's source material.  Whenever an anime doesn't keep up with the source, fans have no choice but to switch mediums and continue the story where the anime leaves off.  If you liked the Negima anime, you had no choice but to read the manga.  If you liked the Sword Art Online anime, you had no choice but to continue the story in the light novels.  And if you liked the Da Capo anime, you have no choice but to read the visual novels.

However, Da Capo's visual novels have no English translation and likely never will.  Despite Da Capo being the greatest visual novel series outside of Key's lineup, it is a very obscure series with few fans outside of Japan.  Since Da Capo started over ten years ago, the fact that the games released in Japan still have no American release or English translation implies no official company will ever take any interest in it.  Mangagamer has done a fine job translating and distributing the core Da Capo and Da Capo II titles, but again, that's only a tiny tip of the iceberg, and to make matters worse, a lot of that material was covered by the anime so it doesn't even help fans get more material than they already knew about.

The Da Capo universe is vaster than the Pacific Ocean, one or two translated games are just a drop in the bucket, as are 5 anime seasons and four OAV's.  For the rest of Da Capo, there's only one recourse:

http://sukebei.nyaa.eu/?page=torrentinfo&tid=100698

Here you can find 'the rest' of Da Capo, short of Da Capo III which might actually get a Mangagamer release some day.

Your targets are:

Da Capo Onsen-hen  (clearly vital!)
Da Capo Plus Communication
Da Capo II Harukaze Ultimate Battle
A.C. Da Capo Rumbling Angel
Circus Disk ~ Christmas Days ~
Da Capo II Spring Celebration
Da Capo After Seasons
Circus Disk Christmas Days 2
Da Capo II Plus Communication
Da Capo II To You
Da Capo II Fall in Love
Kotori Love Ex P  (Da Capo's Kotori gets her own game long route, nice huh?)
Da Capo Dream X'mas
Time Paladin Sakura (Da Capo's Sakura gets her own game, nice huh?)
Time Paladin Sakura 2 (Da Capo's Sakura gets 2 of her own games, nice huh?)

15 games, none of which have ever been translated into English, nor will any of them ever be.  Even if you dismiss some of the silly expansions like Rumbling Angel or Time Paladin Sakura, there are still a tremendous number of plot driven stories that are just as important as the original game they are related to -- To You, Fall In Love, Spring Celebration, Plus Communication 1 & 2, and After Seasons are all just as canon as the original games.

There are a variety of technical barriers that must be surpassed if you wish to actually make the downloaded games play on your computer, but I assume an avid fan is willing to tackle such a challenge or he never would have come this far.  Search online for answers if you don't know them yet.

The remaining real challenge, then, is the language barrier.  All the games are in Japanese, and they will never, ever be in English.  If you wish to be a true fan of Da Capo, your only choice is to learn Japanese.  Or use websites like Google Translate and try your luck on how well it turns out.  One thing you can try to do is skip everything written and rely on your auditory Japanese abilities to at least translate what the heroines say.  In the end, only the heroine's words matter anyway, right?  Since you can click on the message as many times as you want to get their voices replayed, you can intently study each line as long as you need to come up with the solution.  If you combine the voiced lines, the pictures, and the music, you can probably get 75% or so of the game's worth without even learning how to read kanji.

When you combine Suika with Da Capo, the universe just gets that much bigger.  Since Sayaka Shirakawa is Kotori Shirakawa's cousin, the stories both exist officially in the same world.  Sayaka even appears in the Da Capo 1 visual novel and engages in a few lines of conversation with you.  Da Capo's tentacles are so vast that they even appear in Ef.  Chihiro's mouse pad, notebook paper, etc, are totally covered in pictures of Archimedes, the cat from Suika, and Utamaru, the cat from Da Capo.

When I said Da Capo was a once in a decade achievement, this is what I meant.  If you combine all the Da Capo games and all the spinoffs, the overall value of the product is something that is only matched by decades of effort by other great groups -- Key (who, by the way, are the greatest human accomplishment of our time), Final Fantasy, Pretty Cure, One Piece, Magic the Gathering, Warcraft, etc.  In other words, Da Capo is a human accomplishment on par with Star Wars or Lord of the Rings.  ((Actually far superior to both, but at least it gets you into the right frame of mind when these franchises are referenced.))  Just as it would be madness to not listen to Beethoven or Mozart when their music was coming out if you lived in the 1800's, it's madness to neglect Da Capo -- no matter how difficult it is to enjoy it -- when we're witnessing something so great unfolding before our very eyes.  I don't know how much longer Da Capo will last, for all I know D.C. III is the end, but I hope it continues forever.  Like Final Fantasy, Magic the Gathering, Star Wars or Warcraft, it would be best if the legacy just lived forever and more value was added every year.  When people look back, centuries from now, and ask, "What was the 21st century good for anyway?"  I want them to immediately answer, "Didn't you know?  That was when Da Capo was originally published."  That's the scale of greatness we are talking about, when it comes to these fifteen games and counting.