Omen Hawker

The restriction is the entire point: the two mana this produces cannot cast a spell, cannot power out a threat. It exists only to feed activated abilities, the text that lives on the right side of a colon. That narrows the card to a single kind of engine deck: one built around untap-and-tap creatures, artifacts with steep activation costs, and permanents that ask for repeated payments to keep looping. What this offers that a plain Llanowar-style dork does not is the colorless half of its output, which pays generic activation costs as readily as the blue mana pays colored ones, and it does that work a full color away from where ramp conventionally lives. A mana source that can only feed abilities is a deliberately awkward tool: useless to the deck that wants to deploy a curve of threats, invaluable to the deck that has already committed its permanents and needs fuel to fire them again. The 1/1 body is incidental; nobody taps this to attack. It is a converter, turning the tap of one small creature into the activation of something more important, and its ceiling is set entirely by how many hungry abilities are already down and asking to be paid. Print a mana producer with no offensive value and a spending clause this strict and you have described not a ramp card but a battery: charge it into a board that wants power, and it does nothing anywhere else.