What Series Should I Collect Next?

You could do worse than collecting gold or electrum staters of the Bosporus!
I had one years ago, but it was sold as part of a small but nice collection of contemporary coins, when I needed some money. I now regret selling it.

I have to say, I am leaning toward liberty head nickels. I like the idea of starting a late 19th early 20th century series.

I had that feeling.

Ghostrider, I don't know why Peace dollars slipped my mind. I would definitely put that on my potential list.

I suggest you attempt to finish those that you already started. From experience I've found trying to do to many different coins at the same time makes me a bit nutty. I have many lists of coins I need, want, could use, etc. and I purposely leave them at home when going to coin shows. If I did bring a list I'd end up having to sell my house to pay for all the coins on those lists. I too have way to many unfinished sets and will probably never finish them all due to just to many. My problem though is I get Albums free so I'm constantly attempting to fill a new one. You already sound like your getting the same illness that I'm presently in with just to many sets going at the same time.
You really should try to finish more of the ones already started such as the Mercury dimes.
Going back to your finishing of Lincoln Cents. Do you use an Album? Does it have slots for all those silly coins such as the 22 plain, 55 & 72 & 95 Doubled Dies? Are there slots for all the 60's, 70S's, 82's?

The Dansco only has the infamous 1922-D "plain" hole. It does not have holes for the 1955, 1972, or 1995 Doubled Die varieties. However, there are three blank holes on the last Wheat cent page (so that the memorial cents start on a fresh page). Feel free to put whatever you want in them, even if you decide to get those three DD's after all.

I've faced some of this too. I inherited some partially filled albums and had to choose which to finish and which to just ignore. The Roosevelt dimes and Washington quarters were one shy of complete so I finished those first, and since neither thrill me I stopped there on them.

I love Buffs and Walkers so I worked on those next - they were far from done. I ended up upgrading ALL of them anyway to mostly really high quality - and am still working on them. They have run me THOUSANDS THO! So I kinda wouldn't recommend those to most collectors. I have both nice albums and certified SLAB runs in Buffs and Peace dollars as well now.

I found Franklins to be the easiest and kinda BEST. In that for not that much above melt you can typically get really good AU50-MS62 Franklins up and down. Their "Key Dates" aren't that key or expensive compared to commons. You can get some nice AU55s or better even for like $15 or so each - at shops and shows. For the lack of premiums/keys and semi-close to melt I think Frankies are by-far the best/easiest. That they are all 90% is a key contributor IMO.

Peace album was fun but a couple cost me a bundle. So unless you don't mind a couple G4s that isn't a great recommendation.

I like to collect above XF primarily, so Frankies were the easiest BY FAR. The Key guys GET YOU in many albums.

I vascillate between raws in albums and SLABs in runs of 9 coins. Keeps low and high cost coins both in my sights. LOL