Has there ever a certain avatar who was able to bend irregular things like metal, plants, lightning, and if so, could you possibly have an "Ultimate" Avatar that was able to bend every element including the weird ones. I've tried to do research on this but I haven't been able to find much.
All the bendings that you've mentioned in your question are actually subtypes of one of the main bendings.
Plantbending is actually waterbending, the waterbender in this case bends the water within the plants.
Lightningbending is a subtype of bending that comes from firebending.
Metalbending is also a subtype of bending, but this one comes from earthbending.
Lavabending is first regarded as a combination of firebending and earthbending (look for item 112), but in TLOK it was shown as a subtype of earthbending.
The only Avatar that has shown the ability to metalbend is Korra, but this makes sense as only her and Aang had the chance to learn how to metalbend as this bending was discovered recently by Toph Beifong.
There's no evidence of an Avatar performing plantbending and/or lightiningbending, but there are at least two events that indicates that they can, as yourself said, perform weird types of bending:
- Avatar Roku is able to lavabend (during the eruption of the volcano on his home island)
- Avatar Korra is able to metalbend (Suyin Beifong teaches her how to metalbend)
In these two examples, we see two different Avatars performing weird bending techniques.
Avatars are regarded as being more skilfull and powerful than most benders, and if we take into consideration that they have special training for several years (only Aang didn't because of the context he lived in) we can assume that they are able to bend weird elements, which in the world of Avatar are actually subtypes of the main bending techniques.
There's no in-canon answer, as you said, we have no mention of an Avatar that could bend all of these subtypes of bending techniques, thus we can only speculate, but I believe it's safe to assume that a skilled and well trained Avatar can perform lightningbending, plantbending, among other subtypes of bending such as bloodbend.
In Avatar: The Last Airbender, Aang was able to successfully bend all of the basic four elements: Air, water, earth and fire, respectively.
Something new that Aang did was to invent energybending, with which he seized the power of Ozai, Lord of Fire and the father of Zuko who became one of the best friends of Aang.
One is also able to see the same procedure in The Legend of Korra. There is a scene showing a past memory where Aang uses energybending to remove the bending power of a bloodbender.
Korra also successfully bends all the basic elements, plus she was able to bend, more or less, metal.
Hence there is no avatar that was able to bend all the elements you mentioned.