I'm going to provide additional support to an option already suggested here: the BeatStep Pro.
I know this sequencer quite well. My home studio includes two BSPs used for modular, various synthesizers, including Volcas which sound wonderful but have very limited sequencers, or drum modules like the ADX1 which do not include a sequencer. (I also use sometimes an SQ-1 and a KeyStep, I mention this to further support my suggestion of the BSP).
The BSP is very easy to learn, feature rich (random, swing, tap tempo, pattern chaining, memorize, etc), pleasant to work with (very visual, pads are large and responsive, same for the knobs, no diving menu, etc.), two sequencers plus one drum (gate) sequencer, and to top it all off: an unbeatable price. All of these are reasons why so many modularists use it. I don't even see how Arturia could upgrade it.
This does not prevent the integration of certain types of sequencers (or similar) inside the modular for specific uses. For example, as I’m quite focused on random and stochastic, I must say that the BSP will never replace Marbles (*), Stochastic Inspiration Generator or Bloom, and of course all the modules to create random or other specific things (logic, noise, etc. And weird things as Diode Chaos ;) At last, it will not replace the combinations of modules : patching, is still the first reason for the modular.
I almost forgot one little detail: the BeatStep pro is in stock everywhere. A criterion that will now have to be included more often.
(*) There may be an upgrade path here for Arturia and the BSP: see what has been done since Emilie Gillet's work to create the MicroFreak. But I don’t mean that this is desirable, for various reasons...