Here's what I'm thinking, taking this stuff into account.
1) Shorten the time a bit.
2) Allow two on-screen at once.
3) Add a 1.5 second delay between pulling out one bomb and another.
4) If you try to pull out a third bomb, the first one explodes. (This is what the delay is for. Having it any shorter would allow you to sort of "chain" explosions pretty much all over.)
5) Make the Rock Blocks from SMB2 destroyable with bombs, if Two52 doesn't mind having another interactive block. But in the games, explosions don't damage regular bricks or flip blocks, so I don't think they should here.