Suez Canal... I can't really say for whether it might be plausible to construct it in pre-industrial or at least early industrial era but such possibility would hold a huge strategic value and make control of entire Mediterranean sea more important for Western European powers who has colonies east of Suez. It would mean a shorter route for transferring naval ships between Mediterranean Sea and Indian Ocean which is an attractive feature for an European power that need to defend its possessions east of Suez, particularly in India, as well a shorter route for transporting troops from home to colonies there. No doubt that if I wish to build an European empire east of Suez, control of Suez route and a possibility of building a canal would be too important to ignore. This would be a game-changer in building up such empire.
It would be perfectly plausible for the Suez Canal to exist prior to the industrial era, if only because it actually existed in the pre-industrial era in real life. By the 8th century BC, a canal linked Cairo to the Red Sea. But by 1000AD it had become silted up and was no longer navigable. Remains of that waterway were discovered by French surveyors around 1789.
From an engineering perspective, there's nothing terribly difficult about connecting the Red Sea and the Mediterranean. The water level of the two are sufficiently similar. and the terrain sufficiently flat, that no locks are required. Locks are a good idea to reduce the risk of invasive species migrating from the Red Sea to the Mediterranean, but that's not something anyone was seriously concerned about during the EU4 timeframe. In effect, the Suez is a giant ditch.
The great difficulty in building the Suez was due to opposing vested interests, political concerns, and lack of financing. Once British opposition to the canal was overcome, and funding secured, building the Suez was just a question of hiring workers.
In comparison, the Erie Canal was built between 1817 and 1825 with mules, dynamite and pickaxes. It has 35 locks, several viaducts, and a total elevation differential of 565 feet. It runs 363 miles from the Hudson River to the Great Lakes, through marshes, over rivers, and up escarpments.
In short, there's nothing implausible about major canals being built during the EU4 timeperiod.
Many canals still in use today were built in the last quarter of the EU4 timeperiod, because water transport was such an efficient way of moving goods. Even today, despite competition from railroads and trucks, water is the most efficient mode of transport for non-time-sensitive goods over any measurable distance. During the EU4 period, it was the only alternative to overland transport, and overland transport during the EU4 timeperiod was extremely expensive and dangerous relative to transport by ship and barge.
Second, building canals did not require modern industrial machinery; manpower accomplished the same thing, albeit at much greater human cost. The obstacle was usually the sheer expense of getting the project underway, the willpower required, and the lack of real need.
If you block the river zone, you can't through this province.
