Titans in "other" lists were standardized for game balance purposes when GW first put out Epic: Armageddon; the plan was to eventually allow variants and changes, but the big-wigs at GW put an end to that when they cut funding to Specialist Games.
However, there are Titan-focused army lists being developed, all under the Adeptus Mechanicus header. More information can be found here, specifically in the first pinned thread:
Adeptus Mechanicus Titan LegionsThere is hope that "variable" Titans will eventually be point-costed/balanced for use in other lists, but it's a long, hard process.
For Gargant variants, there's the
Orkamedies Gargant Big Mob v2.2Take a look and let us know what you think!