Have a desktop app without having to pay, and/or use a third party app.
Universal app that does SMS, and data based messaging without having to pester people to download an app.
This has been my single most annoying thing since moving to Android.
Hangouts can actually do these things, but Google has (seemingly) dropped pushing hangouts because they're trying to promote RTM or whatever the universal rich messaging protocol is. With Project Fi, Hangouts would do SMS or Hangouts message w/ integrated contacts, rich messaging or simple SMS, and it worked on any device that supported Hangouts (so, basically every web-enabled device). But, it always felt like an experiment ad I wouldn't recommend anybody look into Hangouts for unified messaging now, especially as it seems Hangouts is on the verge of transitioning to a Slack/Teams competitor.
I'm with you, when I switched from iOS to Android I missed iMessage. But then I also realized that iMessage was never working as well as I assumed it was when I was on iOS, and other things like iMessage only being available on OS X came to grate on me (because while my home laptop is a Mac, my development desktop is Windows, and at work I use Windows + Linux [and also have a Mac]).
That said, Google's willingness to start and shutter projects and start competing projects with their own software has always baffled me from a user's perspective.
What's really sad is that Allo is a brilliant little messaging client with incredibly cool features, but fuck if I'm ever gonna tell anybody to use it, and I have no confidence that Google wont' just introduce another messaging platform in 2020.