When after a few more years of training in which you can claim a skill level approaching your instructor(s) and can challenge them to an amateur 3-5 2 minute round match , then it may be time to continue to a second art. Ju Jitsu, Judo, Hapkido (Korean grappling) etc. would make excellent choices to combine with pubching and kicking skills, at which Muay Thai is second to none. A great fighter is never born overnight, so do soak in everything your current school has to offer and don't be a style jumper, dabbling in this and that before you become proficient at any. You'll only wind up in disappointment.