I kinda feel like it'd be cool to have t13 be a culmination of everything before it. It kinda is, but I think it'd be a nice challenge if these aspects were a bit more obvious. I haven't cleared it yet, but I'm on the last phase.
In any case, my idea for t13 savage:
From T1: The adds phase should have a Caduceus type add that splits into two and must be tanked away from each other. It can be fed gusts of meracydia to avoid getting buffed. It will have lots of HP and appear after second divebombs. Must be killed at same time after split and will probably be alive for third divebombs unless DPS is superb.
From T2: One of the three tethers from flare star (forgot exactly what it's called) is cursed, and gives Allagan rot. This only happens with first set of tethers, but the cursed one is unknown. Rot expires after 1 minute, and lasts 15 seconds before exploding.
From T3: LMAO.
FRom T4: I feel like this is already present because of the gusts potentially being eaten and the stoneskins, but seeing a pox like ability would be neat.
From T5: Storm of Meracydia now has twister abilities. Tempest Wing is immediately followed by twister. Every other cast.
From T6: One of the adds must be treated like a bee. To get rid of Debuff, target has to hit mega flare tower.
From T7: Evil Eye now gives out cursed voice. Bahamut gets this ability in last phase.
From T8: During Flare Star, an effect similar to allagan field goes out. That person can not pick up tethers for that round.
From T9: During first phase, there is fire, ice, and thunder adds (so the elemental dance has to be done). The cycle will only occur before mega flare with extended times. These adds are the adds that will be killed in the flare star phase.
From t10: During adds phase, prey will be a thing.
From t11: During first phase, Bahamut uses an attack similar to secondary head where a tank must swap Midcast.
From t12: Three new dragon adds are added in add phase that essentially function like Bennus.
On top of that, the usual. More health, more damage dealt, etc. after Bahamut is killed, his HP is restored ten percent and he charges Teraflare. Just a tight DPS check here, nothing else going on, got 30 sec. This is to ensure everyone is up and managed TP/MP well because without everyone outputting their fullest he will finish the cast.