I've got quite a bit of outboard gear paired up with the MPC One and ... it's been a journey, for sure
The MPC One completely covers sampling and sequencing. Having another groove box just complicates things, since you then have to manage project files on 2 devices and control your rig from 2 places. Also you're paying for functionality you already have, so better use the money for gear that does things the MPC does not do. So for me an MC-101 or the Electron boxes are hard to justify, given the added complexity they add to the setup and the workflow for not offering that much the MPC can't do.
Digital synthesis is also something the MPC covers, at least to some extent. What makes a lot of sense is to buy a Wavetable synth, for example, if you want that sound.
I went mostly for analog gear, it's just a lot of fun to patch and tweak, even if it's not the most efficient way to get sounds out. Analog gear pairs well with the MPC with its CV outs and the ability to sample stuff quickly and easily. I always recommend the Behringer Neutron, it's cheap, sounds great and has a ton of stuff built in, and you can route it into the MPC for either sampling or slapping some FX on top of it.
What you also need to think of is the infrastructure part. With one analog monosynth for example you don't need much besides a few cables, but it'll quickly get more complicated, the more gear you add. I think it's not worth the hassle if you're buying something that has a lot of overlap with the MPC, both in terms of functionality and in terms of sound. So I'd generally look for something that is very different to the MPC.