NewOrExistingTemplate.json

{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "storageAccountName": {
            "type": "String"
        },
        "newOrExisting": {
            "type": "String",
            "allowedValues": [
                "new",
                "existing"
            ]
        }
    },
    "variables": {
        "templatelink": "[concat('https://raw.githubusercontent.com/rjmax/ArmExamples/master/',parameters('newOrExisting'),'StorageAccount.json')]"
    },
    "resources": [
        {
            "apiVersion": "2015-01-01",
            "name": "nestedTemplate",
            "type": "Microsoft.Resources/deployments",
            "properties": {
                "mode": "incremental",
                "templateLink": {
                    "uri": "[variables('templatelink')]",
                    "contentVersion": "1.0.0.0"
                },
                "parameters": {
                    "StorageAccountName": {
                        "value": "[parameters('storageAccountName')]"
                    }
                }
            }
        }
    ],
    "outputs": {
        "exampleOutput": {
            "value": "[reference('nestedTemplate')]",
            "type": "object"
        }
    }
}