Engineer: Well the Sentrys (Sentinels) are an awesome idea but they need lots of tweaking to be an useful and balanced addition to the game.
Power: Well it would be unbalanced if Sentinels would be able to do damage similar to an player, but they should be able to do at least noticable damage or fill in other support roles, also some of the upgrades (like blasterCannon) aren't really doing anymore damage then their precessors. Some of the names could be fixed as well.
Defense: Well a Sentinel is worthless if it will go down with just 1 or 2 strong shots even though fully upgraded. Maybe accept some PUs as construction ingredient.
Other: Moving the Sentinel (like in TF2) would be even nearly required to make it useful, to be only able to place and upgrade it in the actual spot is pretty darn making sentinels useless because you'll have to build them somewhere close to monsters, then if you do that you will likely get kicked around without sturdy so you can't upgrade it. And if you can upgrade it (player support, sturdy or whatever) a higher level wave will just scrap it in seconds anyway making it only useful for defense or healing in small concealed locations.
Sniper: Having a Sniper is good idea but to be of real worth to the team the damage of the Rifle would have to be some 100x stronger than it is right now, also on the other other hand reduce Fire Rate. Actually just more like the sniper enemy that is in the game right now.
Squad Leader: I got the idea according to the voice samples used with the Vehicle/Supply/Artillery Radio. The Squad Leader could specifically call in these things without items, maybe can call in more Vehicles than just the Manta, as well as turrets, walls and bunkers.
Commander: I can imagine the commander primarily with one strong primary weapon and a bunch of possible buffs for all surrounding players, like temporarily higher damage (reduction), speed and firing speed.
May write more ideas about classes later when I have tested the other classes.
Flare Launcher: As opposed to the upgrade the actual weapon has a second fire mode which is actually quite useful in some situations, if just it had autofire.
Mini-rocket Launcher: Again the secondary mode, the wide spread could be useful if it wouldn't reduce the amount of fired missiles so heavily, make it like 2 or 3 times faster to make it at least somewhat reasonable to use.
Shock Rifle: The shock rifle could be pretty useful but it somewhow always lacks in power compared with other weapons, especially because it's sometimes hard to use the combo in wild situations, and if you do the damage still isn't exactly big. How about changing the existing PUs or making new ones. What I could imagine:
Inferno Blast Beam: Pretty much similar to the Blast Cannon of the Sentinel, would save you the work of trying to make an combo. And given the Sentinel already has this finished it shouln't be too hard to make an PU out of it.
Inferno Shock: Could be the shock-core for the already existing Inferno Beam, pretty much like a normal Shock combo but stronger, maybe a little Reedemer-like.
Mayhem: I saw in the mayhem-weapon upgrades shock rifles with ricochet of both beams and cores, allowing for combos around edges.
Link-gun: The spread only goes to 3 on this server which is of course a lot better than the single shot, but still not as useful, I saw the same PU on other servers going up to 7 shots though with multiple PUs equiped, integrating that into the Link would make the primary mode a but more useful.
The secondary mode could use some upgrades as well but I have no ideas for that.
Lightning gun: Also in the mayhem-wepons I saw an Lightning gun with ricochet fire and a slight spread, something like that would make the lightning gun finally useful for player use again.
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