I had guessed it last week, and if you can recall the sadness you felt from episode 16, you'll know what you'll need to brace yourselves for in CLANNAD ~AFTER STORY~ episode 21.
Ushio is suffering from a condition that's almost identical to that of her late mother, and Tomoya is finding that very hard to deal with. He quits his job to look after her as the condition continues to worsen as time goes on with no visible signs of recovery in sight.
At this point he's willing to do anything for his daughter, but the thing she wants most is another trip with her papa. He does eventually try to fulfil that wish, and starts the long walk to the train station.
However, they never reach their destination as Ushio collapses on the way and later dies in Tomoya's arms. So once again fate has dealt him another cruel card in order to break what's left of the broken man.
After that we're whisked away to the Illusionary World where the girl and her robot friend are facing a similar hand of fate. Although they didn't reveal the big connection between the two worlds in this episode, you might have already figured it out if you were watching carefully enough.
The final moments in this episode were again a flashback of that beginning scene from CLANNAD episode 1 where Tomoya is still contemplating on whether or not it'd be better to let Nagisa live her life without him.
I already know his answer from playing the VN, but how well will KyoAni adapt this for episode 22? It'll be a miracle waiting to happen.