As long as Thunderlord's scales 30% bonus AD vs. 10% AP. Thunderlord's needs to scale 20% bonus AD and 10% AP. Then we can finally have AP champs be viable without just hard countering AD ones. Let's put it this way, 
scales 100% AP and 200% AD on ult. Why would Thunderlord's scale harder than an ult ratio wise?
Papa Slothy2 voteshow about we just remove the AD scaling
Jefftiffy1 votes[{quoted}](name=Papa Slothy,realm=NA,application-id=3ErqAdtq,discussion-id=x1etK82G,comment-id=0000,timestamp=2016-02-15T03:53:51.770+0000)
how about we just remove the AD scaling
That leaves AD casters with nothing to choose for in that tree. It has to have some AD scaling and it has to be greater than the AP scaling because you can get less AD than AP and it does magic damage making it scale worse with AD late game.
Papa Slothy1 votesthe AD scaling on it has caused some problems I think that if it was removed and fervor buffed we could get some sort of middle ground
LordGeovanni1 votesdoing magic damage doesnt make it scale worse late if anything it scales better cause most ad champions do physical damage mostly so when people are trying to itemize against them with armor thunderlords will still chunk hugely because its magic damge and not effected by armor
Jefftiffy2 votesI don't think I even mentioned how that even Katarina who is a damage powerhouse with no CC is not being played even with thunderlord's adding to her normal rotation. This goes to show just how much Thunderlord's favors AD. When one of the easiest to play AP assassins isn't being played even while being manaless and having resets.
The Bearded Bard2 votesIt goes more to show that Kat isn't what she used to be. Her damage early on is so bad that it trickles into her mid game, where she's supposed to be at her strongest.
LordGeovanni1 votesKat has barely changed in the past 2 years whats changed is that most released and reworked champions counter her or her burst
67chrome1 votesMerc Treads god nerffed pretty significantly and are more or less useless, making a lot of oppressive CC champions that pretty much force Merc Treads on anyone to be indirectly buffed. A lot of painful CC is more or less true-damage status now, which is not great for Kat.
Also: MR is stupid high now and Katarina's play-style interacts in a pretty "meh" fashion with Thunderlord's. She isn't exactly calling down the thunder every time it's off CD: so I don't see her lack of play saying much of anything in regards to Thunderlord's.
The Bearded Bard1 votesI'd rather them remove autoattacks triggering it than anything. It was made for people that can combo spells, not autoattackers.
Ralanr1 votesI don't get it either.
MunchCrunchLunch1 votesits hard to balance masteries since they are hybrid. they need to make ap/ad masteries seperate.
Ralanr1 votesThen we'd have arguments like, "AD/AP champions clearly get the better mastery that AP/AD champions." You can already see it when it comes to items.
Damage sources will never be satisfactory to both sides. Because there is always one person who is going to complain about the enemy damage due to their items (if not the champion).
LordGeovanni1 voteswhat they should do is make thunderlords and deathfire touch the damage type of where most of its damage is comming from physical damage for mostly ad and magic damage for mostly ap
Mysticman891 votesI've never actually looked too closely at the potential AP per item slot versus potential AD per item slot, and similarly the cost/AP and cost/AD.
If the potential AP's one can build are roughly 3 times as large as the potential AD's, then that'd explain why the ratios are outwardly different, but potentially the same in theory. Alternatively, if AP is 3x cheaper to build (even if the max possible AP is less than AD), then that would also contribute to differeing ratios being 'fair'. I suspect it's a combination of the two in practice, although perhaps the numbers could use tweaking.
(Also there's at least one edge case in veigar [and thresh I suppose] where one can build up infinite AP, but off the top of my head I don't think theres anyway to stack AD indefinitely [nasus stacking not being ad and just a bonus ontop of his Q], so that might also be a consideration.)
This isn't to say that the numbers shouldn't be tweaked, and 20% bonus AD versus 10% AP might be fairer, but I just want to make sure people aren't over-simplifying and seeing "oh, 30%>10%, imbalance!" and there's definitely math to be done versus it being clearcut from the numbers. Not suggesting the OP is doing so, but I'm sure theres people out there who might fall prey to that sort of thinking.