So the no-mobility mages are getting their item rework soon. So here are some suggestion.
No-mobility mage list
This is a list of no-mobility mages in League: Annie, Anivia, Brand, Cassiopeia, Cho'Gath, Fiddlestick, Heimerdinger, Karthus, Kayle, Kog'Maw AP, Lux, Malzahar, Morgana, Nami, Soraka, Veigar, Vel'koz, Vladimir, Xerath, Zyra.
Some with a little bit mobility: Galio, Gragas, Ezreal AP, Orianna, Ryze, Sona, TF, Viktor, Zigg.
To exchange for their lack of mobility, they get:
CC: Annie, Anivia, Cho'Gath, Lux, Fiddlestick, Heimedinger, Malzahar, Morgana, Nami, Soraka, TF, Veigar, Vel'koz, Xerath, Zyra, Galio, Gragas, Orinna, Ryze, Sona, Viktor, Zigg.
Defense: Annie, Heimedinger (with the danger of his mini-turrets around him), Kayle, Lux, Morgana, Nami, Soraka, Veigar (his stun is a big defensive skill), Vladimir, Galio, Gragas, Orianna, Ryze, Sona, Viktor.
High range: Heimerdinger (W), Kog'Maw, Lux, Vel'Koz, Xerath, Zigg.
High DoT: Cassiopeia, Fiddlestick, Karthus, Kayle, Kog'Maw AP (although AD version does better), Morgana, Vladimir, Zyra, Ezreal, Ryze (can also consider burst), Viktor.
Let's start with what we can do to improve their gameplay.
Increase what they do best
So what kind of things should we improve that can help the no-mobility. Obiviously, changes that doesn't affect mobility mages much.
Damage is out of the list because it helps everyone.
CC and defense can be considered as they can help both, but with some fixed condition, its flavour the no-mobility more.
Range is one of the key thing that can be consider, and again, must be useful for no - mobility mages only.
Passive: Whenever you cast a skill, you create a 300-radius zone around you for 10s. This area reduces enemies MR by 10. If you cast a skill on a friendly zone, the zone increase its radius by 50 (keep its center) and increase MR reduction by 5. This stacks up to 5 times. (Max: 35 MR reduction, 550 - radius zone).
This new passive helps the no-mobility mages a danger zone for the enimies. It can also protect you from burst damage from mobility mages with the MR that AS always have. Tanks can also use this item.
+ 60 AP
+ 10% CDR
Passive: Every 15s, your next ability gets 20% increase range. This cooldown reduces twice fast if you stand still.
This ability is good for almost everyone. That's why, only some abilities that can use the effect.
1. It can't be a dash, blink, jump, ... ability (include Graves' R and Cait's E). However, skills like Lee Sin's first Q or Zed's W can use the effect. The reason is: this skill is made for no-mobility mages; with larger range, mobilities mages and assassin is more deadly.
2. It can't be a toggle or passive affect.
3. It can't a channel spell. Some channel spell is very strong with increase range like: Katarina, Galio, Vel'Koz.
Item that aggainst mobility champ
Sometimes, do something better isn't enough. Sometimes, you need to make weakness for the others.
This time, it is CC.
I really like Poppy's W: it is a spell that can counter mobile champions by CC them. It is a good one, because it doesn't effect the no-mobility champion. So here is my idea.
+ 60 AP
+ 400 Health
Passive: When your health below 30%, you create 3 barriers for 5 seconds at the area you standing (similar to Thresh's R). These barries slow enemies pass by while also knockdown enemies who dash through it. When an enemy champion touch a barrier, it break.
Another version is to create a silence area. But it seems to be very OP.