Very simple suggestion but one that would make a big difference. Right now some kits are completely useless in certain item types, like egg armour in OP, or the diorite kit in OP, and egg armour is way too good for hardcore.
I feel like all the kits were made based around the item costs in normal mode.
[EDIT] So I believe that some kits should have more or less depending on the item types, eg moneybags comes with more iron on hardcore than it does on normal.
In NORMAL mode
Moneybags: 10 iron
Egg armour = 4 iron but saves time for people slow using the merchant
Shell cracker = 7-10 iron (in my opinion) (also saves some time)
Feather feet = 1 iron BUT very good in solo when the slightest fall damage can make a difference
Egg shell = Honestly its only 8 blocks and doesn't take much longer to break than sandstone but considering that only 6 iron value
Healer = 4 gold but can save waiting at a gold gen which is very helpful
Buffer & Debuffer = These are the kits I haven't used extensively but in my experience they're very situational, they don't help early game which can be vital if playing against good players on solo or having bad teammates on team. But they can certainly make a difference late game. The regen pot in buffer is a last resort in my opinion once out of golden apples whilst debuffer is great in late game combat and can help you win.
In OVERPOWERED mode
Moneybags = 10 iron (which can get 30 blocks, enough to get to mid on a lot of maps). I believe this should be nerfed to 6-8 iron when on op
Egg armour = Completely useless on op. Needs to be buffed to at least protection two leather
Shell cracker = This just depends on how other players are playing. On most maps obby is easy to get on OP so this can be useless in certain situations but I actually feel its balanced for OP considering the purpose of this kit.
Feather feet = Useless, prot 5 leather boots for 1 iron provide more fall damage protection
Egg shell = Another bad kit on OP, 8 diorite is petty compared to the 30 blocks moneybags could get you.
normal though.
Healer = Half the value that its worth on normal but still balanced I think.
Buffer = On OP I believe this should be buffed to AT LEAST a splash potion because usually you can regen to full if you have armour on and a full hunger bar
Debuffer = The slowness can prove invaluable in this kit for OP mode, same as in normal and hardcore but the weakness is pretty useless in OP, but the slowness makes up for it.
In HARDCORE mode.
Firstly I know many people hate this mode but it can be really fun on team! Provided you have a good team, hardcore can be the most exciting gamemode. But anyway,
Moneybags = 10 iron, balanced for hardcore mode considering the price of tools/weapons/food/blocks, I believe this should be upped to at least 20, (That will get you 20 blocks, or 6 carrots, or a piece of leather gear)
Egg armour = This kit is one of the most unbalanced on hardcore considering it gives the value of 80 iron and you won't get an armour upgrade for a while on hardcore mode. At a level 2 iron gen, this will save you 40 seconds, about 30 though when you consider other things.. A lot better than most kits on this mode.
Feather feet = Fall damage can lose you vital hearts on hardcore mode so this is a good kit. Compared to egg armour it's bad but egg armour is simply too imbalanced, I believe this kit is fair and balanced.
Egg shell = Honestly just bad. This should be upped to 10-15 diorite in my opinion
Healer = Pretty insane on hardcore considering it's worth 20 gold and can help loads in fights, puts the buffer kit to shame because golden apples can be used mid-fight whereas the buffer will just heal your opponent slightly as well as you. Should be nerfed to 1 golden apple in my opinion
Buffer = Again, same as OP, I believe this should be a splash potion, not useful in much, if any situations when golden apples exist.
Debuffer = Great kit for hardcore, not good early game but amazing later game or in important fights late game when you have little amount of golden apples, or any at all. Should stay the way it is.
This is purely my opinion and to be completely honest, I mostly play on teams (and 1.9 if that may make a difference) and I didn't really test buffer on solo hardcore much either.
TL;DR All the kits seem to be built with normal mode in mind, many are unbalanced on hardcore or OP.
This will probably go unnoticed but if you do see it, I'd love to hear your opinion as well!
[EDIT] This could also be included in skywars, but I have not played enough skywars nor do I have much of the kits to make a full suggestion.
I feel like all the kits were made based around the item costs in normal mode.
[EDIT] So I believe that some kits should have more or less depending on the item types, eg moneybags comes with more iron on hardcore than it does on normal.
In NORMAL mode
Moneybags: 10 iron
Egg armour = 4 iron but saves time for people slow using the merchant
Shell cracker = 7-10 iron (in my opinion) (also saves some time)
Feather feet = 1 iron BUT very good in solo when the slightest fall damage can make a difference
Egg shell = Honestly its only 8 blocks and doesn't take much longer to break than sandstone but considering that only 6 iron value
Healer = 4 gold but can save waiting at a gold gen which is very helpful
Buffer & Debuffer = These are the kits I haven't used extensively but in my experience they're very situational, they don't help early game which can be vital if playing against good players on solo or having bad teammates on team. But they can certainly make a difference late game. The regen pot in buffer is a last resort in my opinion once out of golden apples whilst debuffer is great in late game combat and can help you win.
In OVERPOWERED mode
Moneybags = 10 iron (which can get 30 blocks, enough to get to mid on a lot of maps). I believe this should be nerfed to 6-8 iron when on op
Egg armour = Completely useless on op. Needs to be buffed to at least protection two leather
Shell cracker = This just depends on how other players are playing. On most maps obby is easy to get on OP so this can be useless in certain situations but I actually feel its balanced for OP considering the purpose of this kit.
Feather feet = Useless, prot 5 leather boots for 1 iron provide more fall damage protection
Egg shell = Another bad kit on OP, 8 diorite is petty compared to the 30 blocks moneybags could get you.
normal though.
Healer = Half the value that its worth on normal but still balanced I think.
Buffer = On OP I believe this should be buffed to AT LEAST a splash potion because usually you can regen to full if you have armour on and a full hunger bar
Debuffer = The slowness can prove invaluable in this kit for OP mode, same as in normal and hardcore but the weakness is pretty useless in OP, but the slowness makes up for it.
In HARDCORE mode.
Firstly I know many people hate this mode but it can be really fun on team! Provided you have a good team, hardcore can be the most exciting gamemode. But anyway,
Moneybags = 10 iron, balanced for hardcore mode considering the price of tools/weapons/food/blocks, I believe this should be upped to at least 20, (That will get you 20 blocks, or 6 carrots, or a piece of leather gear)
Egg armour = This kit is one of the most unbalanced on hardcore considering it gives the value of 80 iron and you won't get an armour upgrade for a while on hardcore mode. At a level 2 iron gen, this will save you 40 seconds, about 30 though when you consider other things.. A lot better than most kits on this mode.
Feather feet = Fall damage can lose you vital hearts on hardcore mode so this is a good kit. Compared to egg armour it's bad but egg armour is simply too imbalanced, I believe this kit is fair and balanced.
Egg shell = Honestly just bad. This should be upped to 10-15 diorite in my opinion
Healer = Pretty insane on hardcore considering it's worth 20 gold and can help loads in fights, puts the buffer kit to shame because golden apples can be used mid-fight whereas the buffer will just heal your opponent slightly as well as you. Should be nerfed to 1 golden apple in my opinion
Buffer = Again, same as OP, I believe this should be a splash potion, not useful in much, if any situations when golden apples exist.
Debuffer = Great kit for hardcore, not good early game but amazing later game or in important fights late game when you have little amount of golden apples, or any at all. Should stay the way it is.
This is purely my opinion and to be completely honest, I mostly play on teams (and 1.9 if that may make a difference) and I didn't really test buffer on solo hardcore much either.
TL;DR All the kits seem to be built with normal mode in mind, many are unbalanced on hardcore or OP.
This will probably go unnoticed but if you do see it, I'd love to hear your opinion as well!
[EDIT] This could also be included in skywars, but I have not played enough skywars nor do I have much of the kits to make a full suggestion.