I agree with each guitar having a purpose. I've refined my collection to four main guitars that fill their own niche: HBII for those semi and hollow guitar tones. The S2 Singlecut Standard is my solid body Gibson proxy. Custom build Tele for, well, Tele sounds. Custom build Strat for quacky stuff of course and it also has a great Hipshot bridge for whammy action. I have two other guitars I keep mainly for sentimental reasons. They simply aren't as good as the others, but they bring a smile to my face once in a while and it is handy to have something for the occasional alternate tuning.
I really do wish I could be a one (or two) guitar kinda guy, but I'm fickle, so I play what suits my current mood. Switching it up inspires me. There's enough crossover in my main four that at a gig I can have a good backup, or a second guitar for tonal variety, or if my fickleness takes hold during a gig I can switch.
I did feel guilty for having stuff I didn't play, so even though I tend to get attached to my gear, I sold off a few. If you gotta force yourself to play something, you need to move it on to a new home!
The Singlecut would be a good complement to the Custom. They both do things the other doesn't do. But, if you're totally happy with the Custom, go for an acoustic. Everyone needs a good acoustic. You can keep the SE as a backup, or maybe use it to practice on if you don't want to dirty up the strings on the core before a gig, etc.