I'd go a step further - at some point, there will be NO possible new antibiotics to develop or design. At the core, the concept of antibiotics depends on bacteria having a sufficiently different metabolism than our own cells, so that some compound disrupts internal processes of bacteria without affecting the similar process of 'good' cells. We have found a couple such metabolic pathways, and have various drugs targeting them. The problem is that once bacteria become drug-resistant, i.e., mutate so that they use a bit different chemistry, most likely more similar to our cells (obtained from our own DNA?) - then you can't do that anymore.
Perhaps there are some new antibiotics to be found, perhaps there aren't; but in any case there's an end of the road somewhere. After that, only radically different mechanisms would work - vaccines, phages, nanobots, something else, but not antibiotics.
There are plenty of bacterial targets that we haven't even started to explore. Many of the current antibacterials work by interrupting cell wall synthesis. There are many different protein involved in cell wall synthesis. Stop anyone of those from working and you have a new antibiotic mechanism.
Perhaps there are some new antibiotics to be found, perhaps there aren't; but in any case there's an end of the road somewhere. After that, only radically different mechanisms would work - vaccines, phages, nanobots, something else, but not antibiotics.