If anything i think they should buff the trade posts to make them like mini trade companies. You should be able to build trade posts in foreign lands without owning it and there should be some investments attached. Like trade companies it should benefit the owner of those provinces. This was the case in history as well AFAIK. And this change would make them viable without expanding much.
Of course to build trade post in a province, there should be some requirements like money, great relations or alliance etc.
Of course to build trade post in a province, there should be some requirements like money, great relations or alliance etc.