How can we improve WHMCS?

WHMCS tables with FOREIGN KEY restriction

something like this table:

CREATE TABLE IF NOT EXISTS DB.hosting (
id int(10) unsigned NOT NULL AUTO_INCREMENT,
customer_id int(10) unsigned NOT NULL,
package_id smallint(4) unsigned NOT NULL,
webserver_id smallint(4) unsigned NOT NULL,
mailserver_id smallint(4) unsigned NOT NULL,
mysqlserver_id smallint(4) unsigned NOT NULL,
backupserver_id smallint(4) unsigned NOT NULL,
status enum('notValidated','Validated','Active','Blocked','Reserved','Suspended','pendingCreate','pendingDelete','pendingUpdate') NOT NULL DEFAULT 'notValidated',
billing_cycle enum('1','3','6','12','24') NOT NULL DEFAULT '3',
domains smallint(3) unsigned NOT NULL DEFAULT '0',
subdomains smallint(3) unsigned NOT NULL DEFAULT '0',
website_aliases smallint(3) unsigned NOT NULL DEFAULT '0',
disk_space_allocated int(10) unsigned NOT NULL DEFAULT '1',
website_disk_space_used bigint(20) unsigned NOT NULL DEFAULT '0',
mail_disk_space_used bigint(20) unsigned NOT NULL DEFAULT '0',
mysql_disk_space_used bigint(20) unsigned NOT NULL DEFAULT '0',
daily_data_transfer_allocated smallint(4) unsigned NOT NULL DEFAULT '1',
daily_data_transfer_used bigint(20) unsigned NOT NULL DEFAULT '0',
monthly_data_transfer_allocated smallint(5) unsigned NOT NULL DEFAULT '1',
monthly_data_transfer_used bigint(20) unsigned NOT NULL DEFAULT '0',
email_accounts smallint(3) unsigned NOT NULL DEFAULT '0',
ftp_accounts smallint(3) unsigned NOT NULL DEFAULT '0',
mysql_databases smallint(3) unsigned NOT NULL DEFAULT '0',
mailbox_quota smallint(5) unsigned NOT NULL DEFAULT '1',
php_as enum('apache','fast_cgi') NOT NULL DEFAULT 'apache',
cgi_support enum('0','1') NOT NULL DEFAULT '0',
notes text,
comments text,
creation_date datetime NOT NULL,
updated_date TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
expiration_date datetime,
grace_period smallint(3) unsigned NOT NULL DEFAULT '0',
cron_output_mailto varchar(255),
auto_delete enum('0','1') NOT NULL DEFAULT '0',
auto_renew enum('0','1') NOT NULL DEFAULT '0',
PRIMARY KEY (id),
CONSTRAINT hosting_ibfk_1 FOREIGN KEY (customer_id) REFERENCES customer (id) ON DELETE RESTRICT,
CONSTRAINT hosting_ibfk_2 FOREIGN KEY (package_id) REFERENCES package (id) ON DELETE RESTRICT,
CONSTRAINT hosting_ibfk_3 FOREIGN KEY (webserver_id) REFERENCES webserver (id) ON DELETE RESTRICT,
CONSTRAINT hosting_ibfk_4 FOREIGN KEY (mailserver_id) REFERENCES mailserver (id) ON DELETE RESTRICT,
CONSTRAINT hosting_ibfk_5 FOREIGN KEY (mysqlserver_id) REFERENCES mysqlserver (id) ON DELETE RESTRICT,
CONSTRAINT hosting_ibfk_6 FOREIGN KEY (backupserver_id) REFERENCES backupserver (id) ON DELETE RESTRICT
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=latin1 COMMENT='Hosting Account';