Abandoned projects?

Not really in the strict sense but I've been putting some skills on Rhajat with pretty poor IVs (+Def -HP) and then I pulled one with +Spd -Def. I have been stalling a bit since I find Distant Defense to be quite desirable fodder so I don't think I'll merge them.

Oh there's actually Athena whom I've kind of benched after starting to focus on Hana. Not that I'd given her that much inheritance. And Sophia now that I think about it, since Lilina just seems a better alternative with the Forblaze now.

Lyon I've sorta set him up for TA with his legendary tome but he's perpetually stuck at 4* cause there are better things to probably use feathers on.
Also 4* Ursula I've put a little more into her even have an 4* Odin saved for Blarblade+ eventually but now I pulled a +atk 4* L'Arachel who I haven't invested much in yet but long term is probably more worthwhile to use before Ursula.

When Guard Bow+ became a thing, I 5*'d a Niles so he can become a super good mage tank... as time progressed, I lost the will to continue the project, so now he's sitting on the bench collecting dust

As for if it's worth it to continue, that's up to you. Fallen Takumi has a super intersting gimmick that activates on the third turn. Dealing 10 damage AND activating a Panic effect is super useful, but, again, gimmicky. If I had spare feathers, I'd do it, but I'm kinda tight. I have Donnel's that I need to 5* for merge fodder, I have a Hana that I want to 5* AND give her Firesweep S+ AS WELL AS a Lon'Qu (once I get one that's +Atk or +Spd).

Not to mention I'm thinking of doing a merge project for Fir.

I'm really tight on feathers. XD

I would suggest you at least build one Fallen Takumi, Skadi might seem gimmicky but the thing is, this thing is really good to tackle down PVE content where most of the time the enemy tends to stand still until you get within their attacking range, or when the map set-up revolves around the enemy receiving a lot of buffs (Saias's GHB or Infernal Camus), on turn 3 they get Panic Ployed that isn't based off the user's HP.

Nowi, Adult Tiki, and Effie were some abandoned projects. I invested a bit into them and they are 4* +3's at the moment, but I felt like they would have received the short end of the stick at the end of the day being stuck as a 4* raroty, so I didn't really feel like I would do them justice.

I was thinking of making a quadsuna, but after a while I lost the will to build her (also her atk stat kinda put me off)

You should finish him. He is a great looking character, and I just gave him Poison strike, Savage Blow and the Savage Blow seal. He is like the ultimate chipper. Actually always does quite a bit of work in Arena Assault for me.