Ragnaros by Cyous

Ragnaros, in this concept, plays similar to the tone of Azmodan and Abathur.  You have the option to really push lanes, or soak EXP via spreading out your Sons of Flame. 

Ragnaros cannot move.  He must use Submerge to get around, making him very vulnerable to various assassins.  Submerge is a quick cast, but is interrupted by stuns and silences (Abathur's Deep Tunnel basically).  Your playstyle should reflect an extremely aggressive Abathur, in terms of positioning.  Your conservative playstyle may reflect that of a triple/double-lane Lost Vikings.

In terms of gameplay, you'll likely want to push a single lane early.  You sit behind your gate, spawning Sons of Flames.  Eventually objectives happen and Heroes shift positioning.  As Ragnaros, you do not have any meaning teamfight options until lvl10.  Assuming you keep pushing a lane, you spawn a Son of Flame in each lane and Submerge in front of the enemy gate.  Now you would unload Lava Waves into the structures, causing massive Siege damage.  (Lava Wave is wide and has a fairly long range.)

Ragnaros as two very different Heroic abilities:

"World In Flames" causes you to completely destroy a lane of minions, deal moderately-high damage to enemies caught in the path, and causing extremely heavy damage to structures.  You sacrifice your ability to truly teamfight and gain a very powerful tool to force strong minion pushes.  While your team may stall the enemy, you can split-push and force your way through the enemy's lanes.

"The Lord of Fire" is your ability to teamfight, temporarily.  You emerge from the ground and gain new abilities.  With a very high health pool, (even more health than an Avatar-Muradin), you have the damage of an assassin, with the tankiness of a true warrior.  Most of your damage is tied to auto-attacks, making you sacrifice positioning for damage output.  You will want to place Magma Traps behind, or inside teamfights.  Pairing with a team that enables you to stick to a target makes you a truly destructive fighter.  The main sacrifice for this ability is your life ~ you will be extremely vulnerable after the duration ends.