The original drugs were all of one class, called beta agonists. More recently, a second class of drugs, called anticholinergics, has been introduced. The anticholinergic drug, atropine, has been around for hundreds of years. But atropine causes severe side effects, including dry mouth. More recently, a combination inhaler that delivers both albuterol and ipratropium has been introduced that has been shown to be somewhat more effective than either drug alone.
Progress is more likely in two other areas, however.
The first improvement is in duration of action. The first generation of bronchodilators had to be taken every couple of hours. Current drugs like albuterol and atrovent need to be taken every 4-6 hours. A few years ago, a long acting beta agonist was introduced. I participated in a research trial of this drug.