Not for the Riverlands themselves so much as bridging various rivers for ease of travel for commerce purposes. Though you're right in that most of those would be controlled by other Kingdoms.
The issue is that before the Conquest, there was relatively little reason for the kind of interaction we are discussing.
The Vale was utterly secure (save from the North doing some poaching of the Three Sisters, maybe) and also unable to project power thanks to having the Greenfork in the way and fear of the North coming down to fuck them over. They could push their borders to the Greenfork (it is the natural barrier) but that does open them up somewhat to various raiding forces, and pushing beyond the Greenfork means dealing with the river crossing and then having to secure territory in open land relatively far from home and with relatively limited lines of supply/communication.
The Westerlands had ALL THE GOLD! along with relatively secure territory. Their relative lack of secure port facilities and expose to the Iron Isles and Redwyne Fleet precluded any serious territorial expansion. March south and they run into the Reach and just get crushed with sheer numbers while suffering coastal raiding the whole time. Try to expand into the Riverlands and they are having to do it through a single pass into crappy terrain and get blocked by Riverrun.
The Reach has the Westerlands, Stormlands, Dorne, and Iron Isles that it all has to worry about and no particularly good places to expand into. Not to mention already basically being as large as could be practicable given the technology they had access to. If they tried to expand in any direction, all of the rest would gang up on them because a more powerful reach was a threat to all of them.
The Stormlands had access to the Narrow Sea and so a good trade route to Essos along with a sea borne route to the Vale and (to some extent) the North. If they wanted to expand, it would have been into the Crownlands but that isn't an easy investment to make given Dorne and the Reach to contend with and the investment required to keep the territory conquered.
The Riverlands had relatively good lands and expansion potential into the Crownlands but a combined Riverlands and Crownlands have the potential to actually be a threat to the other powers and was simply way too easy (and wealthy) for all of the rest to invade so they would never be allowed to build up into a serious power. I mean a Vale-Westerlands alliance with the Vale putting a blocking force on the Harrenhall-Dary line to lock the Riverlands forces into the Crownlands while both the Westerlands and Vale go raiding in the Riverlands is just too easy. The North is an ever present threat. The Ironborn as well. And a Riverlands expanded into the Crownlands would also have to contend with the Stormlands and Reach.
Ultimately, what really made the Conquest possible was dragons. They allowed a central authority to rapidly deploy an armies worth of firepower anywhere in the realm and the ability to move that "army" faster than most communications could travel. The Targaryens made war in Westeros essentially impossible thanks to their dragons (at least so long as the Targs were unified). That kind of security is what allowed a more unified and cohesive Westeros as a whole, with substantial trade between the realms and where canals would actually be truly useful.
Theres really no reason for the Riverlands not to have been annexes into the crownlands.
Annexing the Riverlands as a whole into the Crownlands would have been a bad idea. An extent Riverlands acts as buffer between the North, Vale, Westerlands, and Crownlands. The Riverlands are also a bitch to govern.
Realistically, the Targs should have made the North side of the river (probably out to about a mile or so) and the Harrenhall-Dary line into the border.
Isn't Riverrun a key location strategically though?
Sure, depending on the situation. For example, it locks the Westerlands out of basically everything East of Riverrun. It is also fairly safe from anyone trying to be a problem; you have to siege it from three sides to actually siege it. But its not actually well positioned to control the Riverlands as a whole or prevent the other powers (except the Westerlands) from marching armies through the Riverlands.
Minimize Tyrell power and create a domain of their own as large as any of the other Kingdoms?
You do the Crownlands expansion because it provides a relatively easily secured, substantial, power base on the continent. You break the Reach because you have the perfect opportunity to do so with the Gardeners dead and it would otherwise be the most powerful competing power bloc.
Rework the Stormlands-Reach borders to favor the Stormlands as you have Orys there and securing your southern flank. Establish the Tyrells and Hightowers as competing, balanced, powers that are neither in a position to be a great issue. For the Hightowers to be an issue, they need to come up the Mandar to the North with the Tyrell Reach and the Strormlands able to easily raid them along both sides of the river and make that a fools game. For the Tyrells to be an issue, they have the Hightowers at their back willing to fuck them over and they have lost a substantial chunk of the Reach's military power.
Martin seems to think the high middle ages were the same economically as post Rome Europe. Hence you get subsistence farming and other things as the main means by which these lords with domains larger than most medieval Kingdoms maintain their wealth.
Martin just really sucked at anything related to economics, logistics, distance, or travel times.
Having a steadier steam of Valyrian migration into Westeros and bringing their engineering and skills might achieve that?
Aegon keeps his dragons and his magic is expanded and presumably because the field is a little more competitive. He has to be smarter?
The issue isn't really one of tech, it's more one of failure to think long term at all.
I mean if Aegon hadn't been an idiot then the Targs would have wiped out the Arryns and replaced them with a House Targaryen of the Vale (Prince of the Vale perhaps?) and established a second son as the founder of that hereditary house.
Dragonstone would have remained the personal domain of House Targaryen and, at worst, explicitly a domain granted at the pleasure of the Crown to whomever (generally the Heir?). Better off would have been isolating it and hiving it off as not part of the Seven Kingdoms proper and instead remaining the personal domain of the Valyrians/the Dragons. No Priests of the Seven or Maesters allowed, everyone with dragons is fostered there, etc. Basically a secure fallback and the center of dragon and/or magic lore along with the source of the hard core Targ loyalists for things like castle servants, personal troops, etc.
Break the Reach and rejigger its borders.
Burn the Iron Isles to ash, genocide the locals, and move people from the North into the area. Probably wed a Stark to a Targ and establish that House as the ruler of the territory.
Don't let the Lannisters keep the Rock. Seize it as a crown domain and make it the headquarters of the Crown's Bank. You never want to trust an underling with that degree of gold and that secure a fortress.
Before you touch Dorne, go after the Stepstones and maybe even Tyrosh.
You need to break the power of the Maesters and the Faith, so get closer to the North as that brings you the First Men and they are generally more in line with Valyrian beliefs than the Faith anyways. More importantly, the Old Gods aren't a competing power base, the Faith is.
Dragons in the Eyrie basically secures the Crownlands against the North, Riverlands, or Vale being an issue and provides another secure point to retreat to/draw from/ pivot on. It also provides a good, secure, route to bring the Northern forces down if needed.
Secure the Iron Isles and you have a naval base on the West that allows you to easily threaten the Westerlands and the Reach if they get restive without actually being any realistic threat to the crowns core holdings.
Secure the Stepstones and you basically make the Narrow Sea your private lake and have a good base to go after Dorne from.
