What Volfield is guiding you toward is known as a 'telestrator'... in sporting events, it's used to draw directly on freezeframes to illustrate upcoming action, most commonly. It doesn't sound like this is what you're looking for though.
Generally for layouts, people tend to just create PNG images in outside image editing/illustration software, then add them to OBS as Image Sources. OBS supports transparency, so it isn't uncommon to have a foreground image, the camera (with chromakey usually), then a background image. Same with full-frame layouts, containing alpha transparency 'windows' that you can place sources behind. Any image editor that can export PNG with alpha transparency maintained can be used to create these. Photoshop, Illustrator, GIMP, Inkscape, whatever.
Some sites do sell overlay packs with these images ready-for-use, but few are aimed at the professional space; most companies tend to have a staff artist on-hire who can whip up something like that very quickly.
Generally as far as setting up a reusable layout goes, it's a bit more complicated in OBS and no, as far as I'm aware no one is making custom versions of OBS for that.
Most commonly you add your placeholder captures, then for each one right-click in the Sources list, go to Transforms->Edit Transform, and set a Bounding Box, and the scaling method to use. This ensures that each capture will stay in its spot and not resize, even if the captured content is switched to a source of a different resolution.
Beyond that, you'd need to be more specific about what part you're having trouble with; the graphical image element, getting your participants' feeds into the scene and placed, etc?