As you mentioned transactions I'd suggest to start from developing around it. Distributed transactions are pain in the ass due to CAP and Byzantine fault. There are no easy and solid soultions to implement ACID in distributed (read 'microservices') environment. As mentioned by other commenters latency may be issue too. To overcome limitations of monoliths you can use SOA (simply saying - just bigger services) and multitenancy (split by title/region and scale independently)

And one more thing - https://en.wikipedia.org/wiki/Conway%27s_law. I haven't ever seen any breaks of Conway's law, so your development teams and process must be (or eventually become) aligned with architecture.