I suppose you have to think firstly about the type of area your lemurs inhabit in the wild, and any special needs they have tied to this, e.g. Ringtails on sandy scrubby areas with small bushes & trees; Ruffeds more arboreal and need to nest if reproducing; others live in spiny forest or sharp rocky areas, etc. Trees and bushes might be an optimum distance apart to allow a challenging leap between them.
They need space to go out & forage for food so you would need good enrichment opportunities for scattering or hiding food built in as well as the ability to bring in special enrichment activities like hanging feeders. You might want to build in feeders or holders for juice or water bottles. You would need safe & enriching plants, ideally some that the lemurs can eat safely. The animals may need access to soil for the intake of minerals.
You would need to think about the position of the sun to provide both shade & sunbathing opportunities.
You would need a roof or hot-wire to prevent them from climbing out, so the ground plan would need to take this into consideration. You would need quick easy access to electrical supplies in case of animal mishaps with any hot-wire, plus some sort of back-up generator or battery.
Inside accomodation would need heating for winter plus a water supply/ies accessible to all members of the group. You would need the ability to isolate indioviduals or sub-groups for social or vet. reasons. Ideally you would have a choice of accomodation / day dens for when social fall-outs happen and the animals need to avoid each other. You might need some sort of outside annexe for sick / socially challenged (or just plain naughty) animals to be safely separated from others at times. If you need to make staggered introductions of animals e.g. with mixed species exhibits, an area like this might also be useful.
You would need to think about flooring materials, and how easy it would be to keep them clean in the context of chosen substrate & bedding materials. You may need to disinfect inside quarters at times, or renew soil substrates especially in a hot climate - how could enclosure design make this easier? Inside & outside areas need good drainage. You would have to think about pest control - would you need extra deep fencing to keep burrowers out, or a roof to protect from birds of prey?
You might want a dedicated food preparation room, especially as lemurs need to follow diets to some degree and have their food isolated from citrus fruit sources where appropriate.
Double doors for security and some sort of lighting for winter, especially if your lemurs are nocturnal & need to have night access.
Stand-off barriers of an appropriate distance to allow good viewing but prevent feeding & teasing from the public. If you're taking the public in for 'experiences' you need to think about how you're going to do that safely - you might need bigger safety areas between inner & outer doors for safe coming & going, plus a way in that avoids inappropriate or scruffy working areas.
Just some of my thoughts - hope they're of some use to you!