I've thought a lot about this pack for the past week (Nessy and I had a short chat in IRC about it recently) and I have made several decisions that I think will help the pack a lot in the long run. I think the main problems I am facing with this pack are:
1) Starting in rank 3, I've been trying to have a "Central trick" for every level. I don't think this is a good thing because a lot of people don't like trick levels, there are only so many cool tricks out there, backroutes are nearly impossible to seal while keeping the artistic design of the levels, and my favorite levels from the pack aren't trick levels anyways.

2) The final rank is a mess. It's got a few levels with the Art theme, but also a lot of levels that I feel don't fit the theme of the pack. I feel like the final rank should keep with the Art theme to respect the pack as a whole, and the levels I had planned for the final rank should either go in a bonus rank, or a different pack altogether.
3) I don't have enough music planned for the pack, especially in the Renaissance and Modern ranks.
4) My own laziness. By far the biggest roadblock.

My plans for finishing the pack are to stop worrying about levels' difficulties, since my recent contest entries (like WaterWorks) I didn't think were that hard, but people thought otherwise. Not requiring central tricks to levels I think will not only make the levels more enjoyable, but also a bit easier (so fewer people will be scared of the pack). This may make the ranks less consistent in difficulty, but I feel that's not as important as finishing the pack or keeping with the theme of the pack as a whole.
The Modern rank will likely not include video game music as I originally planned.
I think my biggest next step toward continuing the pack is to just listen to some classical music for inspiration. This will simultaneously help me find music and give me ideas for levels to make.