Input on Party System for A Game Concept

Hey, All. So as a creative project I’m working on a rough game concept with mechanics similar to Paper Mario and Child of Light, but with a narrative heavily inspired by Nier and Drakengard (...and Demon’s Souls ...)

Ideally I’d be going for a open world 2D side scroller (or top down) with a mixture of RTS and TBS elements in combat. But what I’m thinking about is How I want the payer to explore the world: either alone or with companions. So here’s some of the systems that I’m thinking of: 

-Single player with no companions, exploring the vast and crushing world with only a few friendly faces along the way, but all on your own. This would eliminate the turn base and open it up to more of an RTS platformig system. 

-Single player with AI companions that can be ordered to attack certain enemies, but don’t get damaged by your attacks. Also able to keep with the RTS elements while still having company that can be used as an exploration and combat mechanic for interacting with the game world. 

Lastly this is the option I’ve been putting the most thought in to, which does use some TBS elements in combat , but with a real time turn system:

In the tutorial level of the game, there would be an option of either going through the game single player or multiplayer. For this version, any Party members you choose (if you want any) are permanent and remain with you throughout the game. You’d have the option to choose from nine, but can only take up to three with you. 

Additionally, You have three potential keys to free party members, but those keys can also be used to unlock rooms in the tutorial that will grant you up to three items for extra power to level you significantly in the early parts of the game. However, if you do decide you’d like some company but would also want extra power for yourself, you’re not limited to an either or; you could take on a party member or two and still acquire one or two of the power boosts. 

Anyway, from a development standpoint, which do you guys think would be the most interesting to see or experience? Any input you’d have on the matter would be greatly appreciated. Thanks.