Word for describing ideas/questions that [truly] stimulate thought
Fascinating is the word I would choose:
from the VERB fascinate
1.0 Attract the strong attention and interest of (someone):
1.1 archaic (Especially of a snake) deprive (prey) of the ability to resist or escape by the power of a gaze:
A question that compels thoughtful research and consideration is fascinating, but I would not put the question Why is "red" red? in that category. Why do we call red things red? would be a step closer to fascinating but would still fall short in my mind. Here is a fascinating question:
If people only use 10-20% of their brains, what is the other 80-90%
The etymology of this word is fascinating and particularly relevant to your question:
1590s, "bewitch, enchant," from Middle French fasciner (14c.),
from Latin fascinatus, past participle of fascinare "bewitch, enchant,
from fascinus "a charm, enchantment, spell, witchcraft," which is of
Earliest used of witches and of serpents, who were said to be able to
cast a spell by a look that rendered one unable to move or resist.
Sense of "delight, attract and hold the attention of" is first
To fascinate is to bring under a spell, as by the power of the eye; to
enchant and to charm are to bring under a spell by some more subtle
and mysterious power. [Century Dictionary]
Possibly from Greek baskanos "slander, envy, malice," later
"witchcraft, sorcerery," with form influenced by Latin fari "speak"
(see fame (n.)), but others say the resemblance of the Latin and Greek
words is accidental. The Greek word might be from a Thracian
equivalent of Greek phaskein "to say;" compare enchant, and German
besprechen "to charm," from sprechen "to speak."
Watkins suggests the Latin word is perhaps from PIE *bhasko- "band,
bundle" via a connecting sense of "amulet in the form of a phallus"
(compare Latin fascinum "human penis; artificial phallus; dildo").
Related: Fascinated; fascinating.
If [baskanos] and fascinum are indeed related, they would point to a
meaning 'curse, spell' in a loanword from an unknown third language.