Hmm, this topic exists. A few things come to mind such as Brave Raideen (Tomino and Nagahama's first mecha series! Supposedly dropped at episode 24 because the next one had cliche drama or something), Great Mazinger (last episode (21) ended on a cliffhanger with a major event happening in 22), Brave Command Dagwon (I think the group just faded out, real shame since they only did the first third and save the last three episodes the series was awesome), Baldios (only the first five eps were subbed and it is a dialogue heavy series), and Dancouga: Blazing Epilogue (someone did the first episode and there are only three left, it looks easy to finish). I'd mention Daltanious since it has been over a year, but I don't want to tick off Mr. Lurrah (no offense, sir).
Shinzo Ningen Casshern (Neo-Robot Casshern)
^I second this guy, very fun series.