Here's some ideas
1. Increase movement speeds of both you and the zombies, reduce zombie health but increase the number of them on screen to compensate. Let us move while shooting, but enough that turning and running is the best option for escape, no backwards sprinting.
2. Put a heavier survival focus, make it a good idea to clean out and reinforce key points to fall back on when you encounter a large horde. Make it so you can sneak around massive hordes instead of directly intervening all the time. Make it so you have to find maps rather than just update an omnipotent minimap, like Silent hill, with hand made annotations to make it clearer.
3. Keep the map and areas you encounter open for exploration over the course of the game, instead of making it a long trek across a continent. Keep it inside a few city blocks or a multilevel sprawling mansion. Maybe make every floor a sublevel that you can return to to stock up on healing items and weapons.
4. Keep classic and new enemies from Resident Evil among the monsters, and maybe do some cross-splicing, like any mad scientist would do. Las-Plagas infested Tyrants as minibosses for example. Regenerator hordes that require explosive or incendiary weaponry to break up. Lickers that act as stealthy enemies you may wish to sneak by rather than fight head on. The overall feel should be more of a mad science experiement gone wrong, with Frankenstein's monster-esque monstrosities and animal human hybrids. Remember that a contagion style game only makes sense where you have interactions with a lot of non-infected people, and when you spend the game mostly alone, we don't fear infection. Be creative, and make sure the monsters are fun to fight but still feared because your health and ammo are a rare resource.
5. Give us a choice for which character to play as, with different bonuses. Maybe Chris Redfield can kick down weaker doors to escape hordes, Jill Valentine can unlock more rooms to gain extra first aid sprays and ammunition as "the master of unlocking", Leon is more agile and can climb through air vents to get to places and maybe Claire can hack computers to open security doors or commandeer cameras to get a look at where enemies are.
6. Don't try to make the story a sprawling epic, go back to basics. You are investigating a complex built by a subsidiary of Umbrella after the Africa incident and when you arrive the leader of the complex starts the contamination program to stop you and destroy evidence of the facility. You must quickly retreat to the roof where you get a package of arms and healing items. From there you must go down floor by floor and clear out the building to gain access to the information you are after and use the self destruct to avoid spilling out the monsters.
7. Remove money as a resource. It makes no sense that you are using common currency to buy things either from your supporting force or people who seem to be completely unaffected by the virus or monsters. Make it something like specimens or samples you can collect and trade to the security system for experimental guns or gun upgrades.
8. Let us upgrade our character in subtle ways. Give us armor to equip, give us weapon mods, give us skills to use. Reward exploration with unique items and and keep the enemies always testing us. Make sure that going in for melee is something that you dread doing, not for bad controls, but because you are putting yourself at extra risk. Give us some ranged enemies we should have to engage smartly to avoid damage from. Change the combat from just standing like a turret and picking off headshots from a few villagers.
9. Get rid of QTE's, just do it.
