One-Hit Wonder: To extreme proportions - the band only lasted long enough to release two singles: one ("You Get What You Give") was the hit and the other ("Someday We'll Know") wasn't given a fair shake due to being overshadowed by the band's breakup. Today, most people are only familiar with the former.
Reclusive Artist: Big time. When Gregg went back into writing and producing, he dropped off the pop culture radar and we mean way off, with very little known about his current status. Songs of his popped up now and then by different performers ranging from Rivers Cuomo to Boyzone, but otherwise, Gregg just disappeared.
In 2013, he came out of retirement to contribute four songs to the Begin Again soundtrack. In addition to those, he wrote or co-wrote every other song on the album, barring two. Therefore, it was more or less like a whole new album.
What Could Have Been: Alexander previously mentioned that the songs "Life Is A Rollercoaster" and "Lovin' Each Day" were supposed to be the lead singles on the second (never made) New Radicals album, before he gave them to Ronan Keating . Admittedly, they were huge hits.
The same goes for any song where Alexander gives the song to another artist. He leaves his mark on all of them, but nobody can sing them better than him. His most obvious ones, aside from the Ronan Keating examples, were Murder On The Dancefloor (by Sophie Ellis Bextor) and Inner Smile (by Texas), the latter which he did leak a demo of him singing. Fans naturally felt this way about the Begin Again soundtrack, which had a whole album's worth of new Gregg Alexander songs on it (plus two he didn't write), including four that he sings himself (for the first time in years).
Danielle Brisebois' second solo album Portable Life seemed all set to be a big hit in 1999, but was cancelled after promo C Ds were released. The album was briefly made for official download in 2008 before disappearing.