I think a smaller-scale Open World RPG is quite possible in RPG Maker. A Skyrim-sized game could also be done, but would need quite a lot more work. You could probably re-create Skyrim itself actually; you may need to replace the Action Combat System with Turn-Based Combat but the funny thing about Skyrim is that the vast majority of the game is actually very simple - just replicate the world map and towns, NPC placement, and simplified dungeon layouts and you're basically there. The actual quests in Skyrim are very simplistic and the dungeon traps can all be replicated more or less, so only a few story cutscenes and stuff would really present a challenge. If you are willing to scale back your project then a smaller version of Skyrim is very possible.
I'd focus on 3 things:
Open Maps - The key to the Open World RPG is that it is possible to go anywhere right from the start. This is actually very easy to do since gating the player's movement in a logical manner can actually be a lot harder than just making a world map and letting the player walk wherever they want. The only reason most old school RPGs aren't considered Open World is because they try to gate the player from accessing areas with more and more powerful monsters. But if you implement combat through Events, then you can easily add in level scaling by determining the enemy comp after checking the player's level. As long as all areas of the game can be reached, then the player can go to any town they please and take whatever quests they want hassle-free.
Dungeon Layouts - An Open World RPG is going to need a lot of smaller dungeons instead of a few massive ones. Additionally, your dungeons should avoid puzzles that require specific items unless said items are found within the dungeon itself to make sure that the player can complete it no matter when the player stumbles across it. Just place them with some logical consistency with your over-world (i.e. abandoned mines should be in mountains and preferably outside mining towns or ghost towns, wave caves should be by lakes and seas, etc.) and focus more on simple puzzles and generic mechanics like block puzzles, statue puzzles, boulder puzzles, and tile puzzles. Keys and stuff are fine as long as they are provided within the dungeon or by the quest giver(s) that send you there.
Quests - The most critical factor of all. Having a big open map and lots of dungeons means nothing if there are no quests to give the player a sense that there is actually a purpose to doing things in those places. Try to give every town/city/castle area its own quest line to make each region feel a bit unique. It doesn't have to be long, it just needs to provide some nice story that builds up that area's history and people, like a super-religious town with lots of churches having problems with vampires or necromancers due to generations of persecution, or a small town being taxed to death by a monstrous lord in a nearby castle. Factions are another good idea as they can have greater or smaller presence in different areas and give a sense of regional politics by having their plot-lines crisscrossing all over the place. The important thing is to make ALL of the early quests in EVERY plot-line relatively easy and ramp up the difficulty as the plot advances to ensure that the player can start the quest line no matter what level they start at while having challenges in later missions to encourage them to try other quest lines or explore randomly and come back stronger later. If you'd like, having infinite side-quests is possible through the use of Common Events and lots of switches. They will probably be a bit generic but you can easily set up each quest giver to have personalized dialogue for all of your quest types so this shouldn't be too much of a problem.
Focus more on freedom and less on size and you can easily make a fun Open World type game if you want to.
