Rith, the Awakener

The combat trigger reads its yield off a color you choose, counting every permanent of that color across all players, so a board crowded with creatures, enchantments, and artifacts in your chosen color turns a single connection into a small army (lands sit out of the count, being colorless by default, so the multiplier tracks how developed the nonland board is, not how many basics are in play). The optional cost (pay after damage) is what keeps the trigger from being free value: you commit three more mana the same turn you attacked, before you know how the next combat shakes out. That sequencing is the whole tension. This Dragon wants to be the second hit, not the first, swinging into a developed board where the multiplier is fat rather than into an empty early game where you net two or three tokens for the additional outlay. The 6/6 flying body does the structural work of getting damage through unaided, which matters for a creature whose engine is entirely predicated on landing combat damage, and the Saprolings it spits out stand ready to block the following turn. Among the early three-color Dragon cycle it anchored, this is the most explicitly go-wide of the five, a token engine wearing an evasive beater's stats, and the most table-scaling: its payoff fattens with every additional colored permanent on the board.