Maybe this could work with the right regulations
I think this is most likely to go nowhere and if it does happen, it'll be a disaster but...
What if roaming was not only mandatory but the roamed-to network received hefty charges which the phone's network wasn't allowed to pass on to the customer. That would create a real incentive to increase coverage.
Of course, it'd drive up costs (someone needs to pay for the new masts, and for the endless legal challenges to the idea), but could be worth it for decent coverage.
In fact, this will go nowhere as it only really makes a difference in solid-blue constituencies. What is needed is an initiative that improves phone reception in marginal constituencies.