For me, personally, the problem is the "all-knowing" attitude of the frontline, in where it is not unique to the player's perspective. What I mean by that is the frontline takes both mine and the opponent's units into consideration, regardless, if my units know they are there or not.
It took me a long time and many different games to finally realise this was what bugged me about the system. I knew I didn't like it and it felt forced but I couldn't properly explain why. On paper it sounds fine and nothing is wrong but in game it always turned me off. Incidentally it was today while playing Armored Brigade I realised why it bugged me so much. That game has a very simple UI element for: "Your units are being shot at from this area", which is simply a circle on a map, the size of which depends on your intel of the area(unit proximity, type and condition, so non-stressed recon unit nearby leads to a very small circle, a tank fully buttoned up under fire to a very large one.) This is extremely effective and the uncertainty of it lead me to realise that this is exactly what the frontline in SD tries to represent but fails. Imprecise and uncertain information of: "there is something over there" or "we think there is nothing over there."
What I would like is a frontline I can see, based on what my units think is mine and a different frontline that the opposing player sees, again based on what their units think is theirs. These could overlap or not touch at all without the players knowing about it, leading to real no-mans-land and contested territory situations. SD2 bringing back capture points makes this a possibility.
The information for drawing the line should/would/could come from a multitude of sources, seeing an enemy(someone charging over a field), hearing an enemy(tank driving through town), being fired at by an enemy(hidden ambush from the woods), etc. Implementation of this requires a secondary signature check to a unit, that is lower than the visual detection signature check and not limited by field of view. So units could still hear a tank move, even if they couldn't see it but they couldn't hear a stationary AT-gun, unless the crew decided to move it.
Of course you could attach a whole bunch of mechanics to this so that if a unit were to be ambushed from what they thought was safe territory, that could cause a hit to morale(like say airborne troops causing confusion), or the sudden information(being shot at is very sudden!) could cause the unit to consider their area now no-mans-land, pulling back the frontline(which could remove the morale bonus from feeling safe and secure among comrades, similar to current encircled penalty.) Or none of this at all ofc, should it be deemed too complicated. Similarly the capture points could be based on your information as well. You would know which are yours but not necessarily which are the enemy's. Or simply continue to work as they do now and give some free intel, it's pretty minor in scale compared to current frontline.
Why is this still relevant? Because I would pay for a DLC with a refreshed frontline mechanic in it.
The psychotic Finnish wood-elf