Example : A bad investment.
High risk investments.
Anything that promises to pay too much can't help being risky.
Speculative business enterprises.

Synonyms : bad, high-risk, risky

Example : A notional figure of cost helps in determining production costs.
Speculative knowledge.

Synonyms : notional

Example : If someone saw a man climbing a light post they might get inquisitive.
Raised a speculative eyebrow.

Synonyms : inquisitive, questioning, wondering