Marvel Heroes is sort of a hybrid of ARPGs with a few theme park MMO (the occasional raid) and MOBA cash shop elements thrown in. The thing that keeps people playing the two most successful current ARPGS, Diablo 3 and Path of Exile, are there relatively short seasons with progression elements to shoot for. In Diablo 3 the progression system is Greater Rifts with leaderboards for solo, 2-man, 3-man and 4-man runs. For Path of Exile the main progression system is reaching the highest level the fastest, which is actually quite a challenge in itself due to death penalties in softcore and loss of character in hardcore. POE models their progression system on diablo 3 although their leveling system (maps) is much different.
Optional seasons were a proven commodity in diablo 2, which still gets players to this day. It allows you the opportunity to start fresh, which is something that is attractive to new and veteran players alike. Drop rates can be tuned for these leagues to feel more rewarding than they would be if there were no seasons. Upgrading your character is always enjoyable in an ARPG and seasons allow you to do that more regularly.
All season progression, including items and omegas, would carry over to the existing non season ladder when it ends. Non seasonal ladder will always be there and it will be what everyone is playing now.
Progression is more than a simple leaderboard with the try-hards at the top. They are a way to measure your own character power and something to try to improve on from season to season. Solo leaderboards in both Diablo 3 and POE are based on class used, this erases all imbalance issues between classes and is essential. D2 also had a level based leaderboard as reaching the level cap there was very time consuming
Optional seasons in Marvel Heroes would be pretty straightforward. They'd last say 4 months and each player would start with zero omegas and all of the heroes they currently own. The front end would need to be modified to let players choose non-seasonal or seasonal when logging in. Seasons can have unique season-only items or cosmetics as an incentive/reward. People that don't want seasons don't have to play them, they are designed for the people that leave MH because they get bored.
A progression system in MH would be more difficult. Highest omegas wouldn't really work since you can just buy boosts, and frankly it's not really worth listing top omega players. An endless dungeon system of increasing difficulty could work. Whether or not they could hook that into the Danger room, I don't know. But seasons without leaderboards won't work nearly as well as seasons with them. Gazillion would have to figure out the best progression system if they ever add seasons.
TLDR: Optional seasons, if you don't like them stay in non season which is what we already have, provide incentives to players to keep returning to the game as they offer a fresh start and reward of leveling and upgrading gear more regularly. Leaderboards in seasons allow players to measure their progress and give them ways to use the gear they've acquired in a more meaningful way.