An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate
The example I came across for Norman Osborn / Green Goblin came to about $100 and had cards from Spider-Man and many sets outside of SPM
Improvise lets you use artifacts to pay for generic mana while convoke lets you pay for colored and generic mana
Gracing the cover of the most iconic early D&D starter, the red boxthe original art before edits was saved from a dumpster and returned to Elmore decades later, coffee stains and scrawled notes intact his art has been the anvil that many D&D players were forged on
The Skysovereign in particular jumps out as something with immediate impact, low mana cost, and huge board presence
Id recommend trying out Esper Control if you have Jeskai or Azorius Control pieces already, and if your local metagame is filled with Izzet Phoenix and aggressive strategies