If anyone remembers, The Tick #100 was a crossover with Invincible. It was followed by The Tick #101, which had guest-starred Allred's Madman. But #101 ended on unresolved cliffhanger, with Arthur missing. The editorial at the end of the issue said that #102 would come out and guest star yet another notable indie comics character, but it has yet to materialize.

AFAIK, the only new Tick comics made since are FCBD freebies. Did any of them tell what happened to Arthur, by chance?