This question could cover either just the rulebook or the rulebook plus all updated army books. Either way, i have some serious doubts about 7th edition. I'll just cover the rulebook here. Things GW can actually change until they've updated all the army books. I'll cover what i think/hope will happen by section.
Hopefully they will "fix" the description of Strength. According to the current description, Strength represents how strong a model is. Obviously it should say that Strength represents how much chance a model has of inflicting damage. Skill and the focussing of damage are currently left out while they do play a part.
The Base Size and Unit Strength table will be added/updated. Unit definitions can be clarified, perhaps they'll even manage to put all the rules for the different types in one place. Depending on how they organise the book, this section might also cover Swarms and Fast Cavalry and such, though that can be in Special Rules too. In any case, i hope they will give a definition of "Animal" which will be what those Beasts lore spells and Rune of the True Beast and such use.
I don't expect any changes here, perhaps something about when certain effects take place.
Clarified charging rules and perhaps allowing people to march in difficult. I do hope they provide some better rules for terrain, it's far too open now.
Line of sight should be clarified, especially for Large Targets. Stand and shoot will be clearer, indicating what rules apply (multiple shots) and when exactly those shots are resolved, with whay penalties and when stuff panics. Perhaps even something covering auto-hits?
The whole fleeing/pursuit/overrun/panic thing will be clarified, along with casualty removal.
Panic rules that aren't half random, half vague. One range, one unit strength related rule and nothing else. Fleeing units and what rules apply to them clarified as well, the whole banner/fear thing.
Fear might be adjusted to turn autobreaking into a modifier or requiring more than just a single US of outnumbering. I'm not holding my breath.
Then there is of course the situation where inherent rules cancel eachother out. They might clarify somehow that Unbreakable/Crazed/Frenzy/Immune to Psychology doesn't take away other Psychology rules the unit has, has at the start of the battle or whatever.
Little will change. I think the "Parry" rule will turn into a special rule, along with the 2 hand weapon rule. This would make it so much easier and clearer to show what rules affect certain weapons and what don't (hand weapons, pistols, choppas, ogre clubs, wight blades, magic weapons). They could even make halberds and morningstars useful somehow, but i doubt they will. Most weapons from the army books could be in the rulebook, along with some of their special rules, it depends on how GW want to do this.
How characters work with units will be clarified, perhaps they cover the "re-roll rolls that affect the bearer" problem in this section. The 5" rule will be clarified (might work for 'smaller than the size of the unit'). Maybe somehow they finally figure character sniping is bad enough to disallow it so they either change the 5" rule to completely make the character invisible (also for magic), change how cannons work or simply disallow cannonballs to hit US 1 models (pretty please?). The challenge rules can do with alot of updating too. I personally would like to see them removed altogether, along with "look out, sir", but i fear they will just clarify how those D3/6 weapons and such work on champions.
Generals and battle standards
Depending on how GW will do this, they might put some rules that apply to Undead and Daemons here. I could see the "Hierophant" being added here, which would cover Vampire Count Generals, TK Liche Priests and such. The rules applying to Undead and Daemonic Battle Standards can be added here too.
Ridden monsters and templates might change.
Flying Cavalry might be added here plus perhaps something about flyers that fail their charge.
Their position in the unit and what Champions can and can't do exactly (challenges) might be clarified. Maybe Undead and such get some other benefit from musicians to make up for their lack of fleeing.
Loads of clarification here. What can be Killing Blow-ed (), Immune to Psychology and other Psychology (might be moved to that section), Poisoned Attacks and 7's to Wound, Regeneration, Flame/Fire, etc. Lots of things can be added from the army books, like Hidden, Assassin, Hit and run, Cold-Blooded, Undead, Daemonic, Random Attacks/Strength, auto-hits, etc.
Fleeing and auto-rally removed like it is now, maybe characters can shoot too when marching?
Lots of stuff can be done here. Auto-hitting and random hits can be added, Misfire charts made clear, crew/shooting relations made clear, spiking of Hellcannons, etc.
Difficult terrain, characteristic tests (Initiative) and perhaps even challenges. I'm not expecting but hoping the S7 rule will be removed or changed to make sense. Make it work for Monsters and war machines/ranged attacks but not for those countless vampires and chaos champions with great weapons.
Scrapped or expended. Warhammer Skirmish/Borderpatrol will likely be part of the rulebook (like Kill Team) so this would tie in with that.
Oh dear. At the very least i think there will be some restrictions on magic. Dice generated by Wizards can only be used by themselves, a dice per turn limit for Wizards or anything like that. The spell list are sure to recieve attention, i suspect GW will manage to balance them a bit more and even make it clear what the spells actually do. Looking at the WE lore, one might expect they will change the spells to be more supportive instead of just being a variation of D6 S4 hits.
Magic weapons and what rules apply to them will be clarified and hopefully changed so that they count as hand weapons normally (for parrying). The two-handed weapons thing for battle standard bearers will be clarified too.
The "can't cast while wearing armour" rule can simply be removed as we have the restriction that you can't wear armour unless you have the option to do so. Blood Dragons, Chaos characters, Priests and such all simply ignore this rule. The only case where this rule isn't ignored is with that HE Honour nobody ever uses. The difference between armour and shield will be clarified too. I can see helmets being added as category too, so they don't have to repeat the "can be used with other armour and shields" line every time.
The common magic items will be changed to make some sense. Better yet, make them part of the army books so you can do different costs for different armies.
Of course, this is only what i expect to see, not what will actually happen. However, going by the fact that the rulebook must work with all existing army books and that GW like to stick to the main lines of their current system, i think that even if they change things like characters sniping, auto-destroying chariots, restrict magic and balance the spell lists, the game will still be about cavalry and other things that will always do what you expect of them. I don't think GW will change the basic rules to help infantry, i have my doubts they can make them worth it even with updated army books. Clearer rules and more balance, but still about movement, reliability and staple choices.