New races is a bad idea because more races = harder balence and harder to write plotline.
I say ammo / energy for units.
Ammo is free but replenishes only while near your buildings or ally buildings and once out of ammo the unit will bu unable to attack.
Indivudial shots remaining on the unit will not regen but clips (groupes of ammo would).
This would mean that especialy for most terane units that they need to retreate sometime to replenish ammo or else they will become unable to fiire.
Energy is generated always but depending on the power of the attacks the amount of shots stored is different.
Energy is also used for special abilites and in some protos would be able to be shiped to restore their shield power.
Ships like the teran battle cruiser will be able to attack constantly unless they use the beem which will drain almost all power and thus the ship will have trouble attacking due to it having to recharge a bit.
Also a good hero system would be nice but with different unique stats.
Mobility
Duribility
Power
Mobility would be the equivilent of agility in wc3 but instead of adding armor it makes the hero harder to hit because he is able to evade attacks easier.
Also the higher this stat the faster the hero would be able to move.
In the case of heroes using physical attack like zerg heros and zelot like heroes it would also up their attack rate slightly and damage.
Duribility would be the equivlent of strength in wc3.
The higher the value the more armor the hero would have.
The greater the vale the more HP of damage able to be taken.
Also the higher the value the less lightly the hero would be able to be stunned by powerfull hits.
Improving this stat will only add HP regen to organic heroes (zerg) and half the amount to armored organic heroes (marine like hero and zelot like hero) and none to machanical heroes.
Power is the equivlent of intellgence in wc3.
Power affects the energy of a hero and how fast it is regened.
Power affects the shield of all protos heros by adding more regen and capacity to shields (only protos).
Power reduces the effect of enemy energy abilites (things like slow and stuff).
Power improves the HP regen of all machanical heroes and half the amount on armored organce heroes and does not help regen at all for pure organic heroes.
In the case of a zelot like hero Power will both help shield and hp regen although it will also gain hp regen from Duribility.
Some heroes will use ammo to attack, otheres their energy, and some will use nothing to attack (zerg, zelot like heroes).
Depending on the ability each stat will also improve the ability.
For example a Energy net ability which would mass slow would be more effective and last longer with more energy.
Each unit may have more than 2 attacks and might even have 5+.
Each attack can use different ammo although energy for attacks in universial. For example a goliths will use AA rockets for anti air and might only have 60 yet it will use a machine gun for ground which might have an ammo of 600+ then it might have a heavy duty laser mounted on it which would use energy.
Armor could be different from WC3 in that it instead of being a damage reduction amount is a fixed reduction of 1 damage per armor.
Each attack has a minimal damage doable before evasion multiplyer so that a missle could do atlest 8 damage while a machine gun might do only atleast 0.5 damage a shot.
There could be more recources like listed below.
Metal
Mineral
Gas
Biological enhancing crystals
Organic material
Only 4 recources are available to each race.
Mineral and gas are universal and all races have them.
Metal is only used by protos and terane civilizations since zerg have no use for non organic fusable materials.
Biological enhancing crystals are only available to protos and zerg since tereans do not have the knolwedge how to use theses rare and powerfull recources.
Organic material are only used by zerg and teraen races since the protos do not need excess organic materials to grow food or build bases.
A recource system like this would allow diverscity and force people to work for recources since alot of civ specific recources are used in large quanitites and a ally of a different civ will only be able to supply you with 1 needed recource.
Multiplayer might be totaly different from orignal SC.
Heroes levels are uncapped and after each game you earn "account EXP". Once an amount of Account EXP is achieved you will level up and in that case all heroes willstart with the level of 0.5*your account level. Also the higher your account level the stronger your normal units are. Certain spells and spell levels for your heros could require a certain account level.
To stop lamers power leveling an account only bonous exp will be gained from a win in pvp if the oponents exp is greater than 0.
Also if the oponent has 0 EXP the oponent on the other team cant gain EXP.
To stop ubers owning noobs the teams total levels must be even.
In the case of a lvl 90, 90 lvl 1 can fight him but only 7 empres can exist. This is achieved by the 90 lvl 1 being devided into 7 groupes and the total level of each groupe added together to form a empire with a level of the group.
To lvl up more and more exp is needed and to make it more chalenging in pvp 75% of exp gained by the winner would be subtraced from the loser and none of the losers gained exp that game would be added to his acount exp. even in ffa and team games including if there is more than 1 team.
By losing you can lose levels untill you have 0 exp (the minum).
Since PVP does not generate alot of EXP games can be forght agains comps (comp stomps)
The difficulty of the comps effects your exp gain and naturly more comps = more exp.
To stop easy killing of comps the comps levels will match thoes of the chalengers.
No pvp while fighting comps.
The difficulty of the comps would effect their level by a multiplyer of the chalengers combined level devided by the amount of comps.
The AI will improve and the computers might even start cheating ate realy high difficulties.
I could go on for ever but like you guessed. . .
THIS IS ALL BUT A DREAM. . .
A GOOD DREAM, BUT. . .
A DREAM. . .
Hoped you enjoied reading this since I definatly enjoied typing this
.