Just finished watching your fanvid and tbh, considering it's your first one, it's actually pretty cool!
Your intro is fine, my only suggestion there - if you don't mind me suggesting that is - is you have a duration of twenty seconds of instrumental [intro] before the first words/lyrics of the song you've used. The intro [vision] itself is about thirty one seconds long. One option to explore is to fade the start of the instrumental up under the picture of Alt-Livia when it changes to the more desaturated look [around the 10/11 second mark] as there's about twenty seconds of intro vision/pictures still to go before it ends. Then the lyrics start at the beginning of your video proper.
I'm aware the intro scene has music already under it from the actual episode but you can try it out/experiment and if it doesn't work, well... it doesn't work . Intro's can work quite well with quiet instrumentals under. But it's all up to you at the end of the day anyway!
The other voice overs are OK, the best thing is you're trying ideas and that's all good . You're off to a solid start.
With your dissolve transitions consider the durations. Some can be longer, some shorter than they are already - possibly. You feel it out for yourself. Long dissolves are like exclamation marks at the end of a paragraph - they are impactful and sometimes sad. Short dissolves are like commas in a sentence. They help move things along while separating ideas along the way.
The vertical and horizontal wipe effect doesn't work quite as well as the mixes [dissolves] do in this video but that's just a personal thing . Horizontal and vertical wipes work well for comedic action. I think you've also added a flash-to-white or mix-to-white or something like that which works well in this fanvid. Because your music is reasonably gentle and the subject matter is emotional, mixes [dissolves] with cuts judiciously placed, will work AOK though - I didn't mind the push wipe !
Notice also that your voice-overs still have the song under - that's all good . Consider trying - I'm not sure if this is possible with iMovie - dropping the volume on the music track a little when the dialogue starts so that it's not competing with what the characters are saying. This is an old television technique where background sound levels are lowered when a voice track is used [In some cases the voice track may have to be raised slightly, especially if the character is speaking quietly]. The background sound/music is not turned off, it is just lowered slightly, so the voice can be clearly heard, then the music track volume is raised back up again when the voice track has finished. As the next voice starts lower the music again [it must still be able to be heard under the voice] then raise the music up again when the voice finishes. Do this as many times as you need. The process may seem odd but for the viewer of your video - they won't think twice about it - unless you lower the music to the point of near silence or you overdo the number of voice overs!
Ultimately, though, it's your fanvid - you make it how you want it.
Great start. I look forward to seeing what else you're going to come up with. I'd better get off my butt and make another one myself!