I added the Dreadbox Typhon and the Novation Peak to my DigiCombo.
Digitakt and Digitone is such a powerful combo that you could produce music for decades. Study the manuals, experiment, get lost in video tutorials: are so amazing machines!
I say: buy nothing. If you don’t know what to buy, it’s because you don’t need it.
Yes, Digitakt is a samples machine and Digitone is a FM digital synth, so you may want an analogue subtractive machine. A4 is very good, but it is also a wild horse IMO, a little hard to master. Look inside yourself: is a true musical need or is GAS? 
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Wise suggestion…
Did you speak with my wife? 
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I’m not a gear’s master, but I can give you a little advice: if you are using Digitone as the synth and Digitakt as the drum machine you are loosing most of the fun. Both machines can do everything and their interaction is limitless. Specially DT: it’s very much more than a drum sounds source. It is a very well equipped synth, that can be driven in any musical direction.
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I have the Digitakt/Digitone combo along with the AnalogFour MK2. You really need to think hard about what you need and what you feel is missing. If you want multiple analog monophonic parts then go for it. Honestly, the unit I use most with that combo is a Roland MC-707 although the 101 will do a lot for that combo. A single channel drum kit with 16 sounds from the MC will free up a lot of channels on the Elektron devices. The ability to play pads, strings, etc… and not worry about running out of polyphony is a big bonus.