Ideally Monarch need to be pushing their Monarch Holidays brand further and better using their knowledge and expertise they have gained over the years of offering such product as an advantage.
They should also make better use of their Vantage card product as a USP. Perhaps join forces with Avios to collect and spend points on it. Tier points should be earned depending on flight spend / distance flown as per the current card, although with easier to obtain perks closer matching the likes of BAs Executive Club... This is something not really seen in the low cost market but absolutely does attract repeat business.
Finally, with their new Max 8's on the way they will finally have a standardised fleet. The new aircraft should better match demand across a larger mix of destinations throughout the year. I feel while the A321 has been perfect for their summer sun routes and previous charter days, it's also been too big to venture into new scheduled markets outside of this, especially during quieter parts of the year. Yes they have the A320 but again this adds a degree of complexity for scheduling to fit in whuch aircraft can fly to where.
Monarch already have a lot going for them - i feel it's just been a case of them not using these positives to their own advantage. As soon as the new aircraft arrive hopefully we'll soon see new markets and destination pop up on the route map. Alongside their bread and butter routes, perhaps they should have a focus on offering more niche routes from their current bases. As for long haul it's really not the be all and end all. Personally they should stay clear of this for the near future at the very least and completely focus on growing shorthaul and pushing their product.