Opposite words for trivial:
weighty, real, pivotal, meaningful, major, decisive, conspicuous, noteworthy, competent, valuable, of import, substantial, renowned, illustrious, prestigious, eminent, unfrivolous, remarkable, distinctive, overriding, chief, effective, exceptional, potent, fundamental, essential, considerable, profitable, outstanding, famous, distinguished, great, useful, discernible, important, overmastering, striking, momentous, sound, appreciable, basic, powerful, efficient, eventful, all-important, material, fateful, beneficial, adequate, big, worthwhile, consequential, preeminent, serviceable, key, sufficient, principal, worthy, overbearing, fatal, valid, significant, prominent, serious, impressive, expedient, advantageous, notorious, solid, measurable, dominant.
Usage samples:
  1. If it isn't too trivial at such a time, do you mind letting me try them?"
    "Loaded Dice", Ellery H. Clark.
  2. Remember that I shall expect full details, and that no detail is too trivial to interest.
    "My Man Jeeves", P. G. Wodehouse.
  3. But not only should you seek to guard them from the danger just alluded to- your affection for them should lead you to enter into their pleasures as far as in your power to do so; to give interest and variety to the home circle; to afford them, at all times, the assistance of your judgment in matters of trivial as well as grave importance.
    "The Home Mission", T.S. Arthur Edition: 10 Language: English.
  4. There are many of them, some important, others trivial.
    "Cathedrals of Spain", John A. (John Allyne) Gade.