A feature which would make Marketpress compete with the other big marketplace plugins (WC Vendors, WC Marketplace, Dokan) would be support for vendors. What I have yet to see is a plugin which makes it easy for any logged in members to post products (e.g. finn.no, marktplaats.nl) where they can choose to sell it directly on the site through a gateway (with or without commision), or make it possible for customers to contact them and arrange a meeting to make the transaction in person (very useful for used and second-hand products). What is also missing from other plugins is that there is no way to diversify the options or content for corporate sellers and private sellers, this would make this plugin unique and give it a competitive advantage.
The second feature I would like to see in your repository, either implemented in an existing plugin like Membership 2 or as a standalone, would be your own supported "custom menu plugin". Having either the feature of making different menu bars for separate occasions (e.g. logged in or out), or making it possible to adjust the visibility option in the same way as widgets would be a very needed and sought plugin, considered that similar products are dropped and also not up to date.
I love your plugins and I hope to see a lot more from you in the future!