Why spend the time and resources to include Frontline as a gamemode? It is very similar to Push, and I don't see many Push players dying for a new, yet similar, gamemode. What Push players really want (at least what I gather from this subreddit's posts) is better spawn restrictions, which should be much less resource intensive than creating a whole new gamemode. Frontline wasn't even a popular mode in Day of Infamy. Even a year ago, when DOI's player count was slightly better than it is now, I rarely played a Frontline scenario. The most popular modes in DOI are Invasion (Push) and Offensive (Push with an additional respawn mechanic for defenders).
Instead of including a gamemode that wasn't asked for, why not work on gamemodes that have been asked for since launch? Ambush actually has real potential for Sandstorm. Personally, I felt that the Source maps weren't very well suited for Ambush with their size and focus on Lanes (I'm looking at you Ministry! Or Station, or Market, or District, or Revolt), but Sandstorms maps are so much more spread out and allow for tons of possibilities with this mode. There is also a well established playerbase for Ambush from Insurgency 2014 when there isn't an established playerbase for Frontline. It wouldn't even be that hard to implement - put 2 to 3 extraction areas onto a map and let the players figure out how the hell they're going to get there. That's the whole appeal of the mode anyway - the teamwork and coordination it takes to get your VIP to extraction while simultaneously performing false attacks on the other extraction points.
For coop, hardcore checkpoint and frenzy checkpoint are great fun and break up the monotony of the same objective layouts, but they're ultimately still just checkpoint with "mutators". There are so many GREAT coop modes from the Source game. Hunt, Survival, Outpost. And once again, Sandstorm's maps would just feel right when playing these modes. Outpost in Sandstorm would allow larger objectives with more chances for the enemy AI to come pouring in - we already can see that with the objective holding in Checkpoint. Night maps and NVG just scream out for Hunt to make a return. Just imagine going through Summit or Hideout looking for those last two insurgents that you need to kill. Survival? With all the added buildings, open roads, one would actually have to work to survive. Tell from Source was imo the best Survival map just because you could turn a blind corner and run into the enemy. With the openness of Sandstorm's maps, the enemy could be anywhere and give the player the feeling of constantly being in trouble. Survival was just a blast in general too. Starting with just a pistol and eventually working yourself up to a fully kitted out M4, chest carrier with 3 grenades, and heavy armor? Sign me up!