Ok so we know that the sentinel is a secondly class, but I don't see the point in this I believe that they can very easily make it a normal class, while allowing others to equip shields. I can see a lot of benefits to doing this such as adding a new tank option, making a new weapon, etc. So here a idea of how to make Sentinel into a normal class.
Sentinel
-to unlock the shield for other classes you must get Sentinel to rank10 that unlocks a trait that allows you to equip shields on other classes that are 1 handed in the secondly slot.
-You can still rank up Sentinel when its in your Secondly slot, but the exp is 70% less.
-When ranking up Sentinel in the Secondly slot there is a cap on the amount of exp you are capable of obtaining. You can only rank up Sentinel as a secondly skill to halve the rank of the main class. (Example if your rank is 10 you can only get Sentinel up to Rank5 as a secondly skill. So the max you can get it to as a secondly skill at max rank with all the classes is 25.)
Shields
-When a shield is equipped in the main hand your class changes to Sentinel
-Can use all shields
-Tower shields are a new type of shield only use able by Sentinel.
Skills
New skills have a + in front of the rank
+Rank1:Bash (Basic attack)
Rank1:Guard
+Rank1:Rejuvenation (Converts all MP to HP)
(Unlocked main Scenario quest 2)
Rank2:Aegis Boon (500TP)
Rank4:Shield Bash (500TP)
+Rank8:Mediguard (Gives a strong Regen effect, and raise def by 20%, but cannot execute any attacks while under this effect.) (TP:1000)
+Rank10:Shield Bearer (Trait) (Allows others to equip shields in secondly slot)
Rank12:War Drum
+Rank14:Last stand (If the next hit is supposed to K.O you, your HP stays at 1 HP.)
Rank16: Outmaneuver
+Rank20:Heavy Bash (Guild)
+Rank20: Deathward (When HP is at 10%, activation of this skill raises TP to 3000)
+Rank22:Shield Bash II (1000TP)
Rank24:Aegis Boon II
Rank25: Deflection
+Rank28:ManaGuard (Gives Protect + Shell) (TP:1000/MP:10)
+Rank30:Vendetta (Gives a boost to attack, based on current HP)
+Rank30:Protect the King (Guild) (Raises Max HP when protecting another member with Critical HP.)
+Rank34: Deflection II
+Rank36:Mediguard II
+Rank38:Steelguard:Increases Def by 50%, but Lowers Atk by 50%.
+Rank40:War Drum II
+Rank40:Skullbreaker (Guild) (Bashes the enemy, additional effect:Silence)
+Rank44:ManaGuard II (Gives Protect + Shell) (TP:1500/MP:20)
+Rank48:Aegis Boon III
+Rank48:Shield Bash III
+Rank50:Protector (Increases your affinity with Sentinel-based actions by 10, up to a maximum of 100.) (Guild)
+Rank50: Conversion (Transforms all your TP to some HP.)