They could open up iMessage, which they originally said they were going to do, then stopped talking about it entirely. Even if they didn’t want to open it up they could make an iMessage Android app, like their Apple Music app.
When did Apple say they were going to open iMessage? I do recall Steve Jobs saying this about Facetime [1] during its announcement, and that a) it was a surprise to the team building it, and b) was abandoned when Apple was sued for patent infringement by patent troll VirnetX [2], causing them to change the protocol from peer-to-peer to server-based.
It tough to blame Apple for not open-sourcing a protocol they are sued about by a patent troll. Until resolved properly this would be a toxic code-base anyway.
Of course they could restart from scratch, but I guess they have no incentive to make efforts about that. What will be meaningful however is be what they'll do after patent exhaustion date of infringed VirnetX "patents".
Apple just finished closing down (pretty sure) all their stores in the East District of TX so all these suits can no longer be filed there. Might help conclude a bunch of these...or not...it's a ton of money we're talking about.