forge of invention is scary
all you need is a little bit of card advantage and it waives its own mana cost. seems pretty free unless you really care about casting 3 spells a turn
on later turns, it lets you use the extra land(s) the same turn you put a tapped land into play (eg. shockland, temple, gate, gainland, field of the dead, and things like growth spiral / circuitous route / risen reef / golos / green cavalier)
and then it has all these other modes of letting someone spend 15 mana at once. those decks won't be too consistent, but that doesn't mean you won't face them. random stuff like:
- planeswalkers ultimating the same turn they're played because of planewide celebration (4x proliferate) or elderspell
- red cavalier & friends haste attacking for 20 from an empty board
- ral storm conduit or reverberation copying stuff
getting to actually spend mana on activated abilities is just extra. might even be able to slot those in for free if we get creature lands or if food tokens are regularly consumed