Wand of the Worldsoul

If the next spell you cast after Wand of the Worldsoul's ability resolves already has convoke, giving it convoke again doesn't have any real benefit. Tapping a creature you control will still pay for only {1} or one mana of that creature's color.

Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.