You can use a weighted random. That means that you assign a weight to each possibility… Then to compute the probability of one of them, you divide its weight by the sum of all weights.
See How do I create a weighted collection and then pick a random element from it?.
Thus, you would be able to make one possibility (e.g. generating a bomb) more likely by increasing its weight, without reducing the weight of the others. This way their probability would never reaches zero.
By the way, a pretty old trick to increase difficulty is to make the game go faster.
Addendum: An alternative to increase the difficulty by making bad things more frequent, is to increase the difficulty by making good things less frequent. For example, if you introduce power-ups, you can make them progressively less frequent. Some power-up ideas:
- Reduce speed
- Invulnerability from bombs
- Score multiplier