Existe alguma evidência apontando para o desenvolvimento futuro planejado de Arda?

Tolkien pretendia que Arda fosse uma forma antiga de nossa Terra. Das Cartas, Carta # 211

I imagine the gap to be about 6000 years [destruction of the Ring ~ modern ): that is we are now at the end of the Fifth Age, if the Ages were of about the same length as 2nd Age and 3rd Age. But they have, I think, quickened; and I imagine we are actually at the end of the 6th Age, or in the 7th.

Na verdade, Tolkien afirma que os Hobbits ainda existem. Da Sociedade do Anel, Prólogo:

Hobbits are an unobtrusive but very ancient people, more numerous formerly than they are today; for they love peace and quiet and good tilled earth: a well-ordered and well-farmed countryside was their favourite haunt. They do not and did not understand or like machines more complicated than a forge-bellows, a water-mill, or a hand-loom, though they were skilful with tools. Even in ancient days they were, as a rule, shy of 'the Big Folk', as they call us, and now they avoid us with dismay and are becoming hard to find. They are quick of hearing and sharp-eyed, and though they are inclined to be fat and do not hurry unnecessarily, they are nonetheless nimble and deft in their movements. They possessed from the first the art of disappearing swiftly and silently, when large folk whom they do not wish to meet come blundering by; and this an they have developed until to Men it may seem magical.

Como Arda é, de fato, a intenção de ser nossa Terra, pode-se deduzir que ela foi eventualmente industrializada, com ou sem o planejamento de Tolkien. :)

Uma outra coisa: o próprio Tolkien começou a trabalhar em uma continuação do Senhor dos Anéis intitulado > A Nova Sombra , que aconteceu durante o reinado de Eldarion Telcontar (filho de Aragorn). Colocando pelo menos 120 anos na Quarta Era. Tolkien disse:

"I did begin a story placed about 100 years after the Downfall, but it proved both sinister and depressing. Since we are dealing with Men it is inevitable that we should be concerned with the most regrettable feature of their nature: their quick satiety with good. So that the people of Gondor in times of peace, justice and prosperity, would become discontented and restless — while the dynasts descended from Aragorn would become just kings and governors — like Denethor or worse. I found that even so early there was an outcrop of revolutionary plots, about a centre of secret Satanistic religion"

Nós sabemos que ele considerava o tempo dos Elfos passando e os Homens herdariam. Saruman diz isso, de acordo com Gandalf, quando ele originalmente tentou subvertê-lo:

'The Elder Days are gone. The Middle Days are passing. The Younger Days are beginning. The time of the Elves is over, but our time is at hand: the world of Men, which we must rule.'

Mas talvez a melhor pista de como Tolkien pensou no futuro vem na conversa entre Gimli e Legolas quando eles entram em Minas Tirith:

'It is ever so with the things that Men begin: there is a frost in Spring, or a blight in Summer, and they fail of their promise.'

'Yet seldom do they fail of their seed,' said Legolas. 'And that will lie in the dust and rot to spring up again in times and places unlooked-for. The deeds of Men will outlast us, Gimli.'

'And yet come to naught in the end but might-have-beens, I guess,' said the Dwarf.

'To that the Elves know not the answer,' said Legolas.

Você tem que lembrar que não estamos lidando apenas com uma linha do tempo do passado-presente, mas com a linha do tempo do desenvolvimento de suas idéias e do mundo de Tolkien. Você pode vê-lo no Book of Lost Tales, uma das primeiras compilações que Christopher Tolkien trabalhou nos primeiros manuscritos inéditos de seu pai, como você pode ver no prefácio:

The Book of Lost Tales was the first major work of imagination by J.R.R. Tolkien, begun in 1916-1917 when he was 25 years old, and left incomplete several years later. It stands at the beginning of the entire conception of Middle Earth and Valinor, for the Lost Tales were the first form of the myths and legends that came to be called The Silmarillion.

Aqui, Tolkien imagina um marinheiro inglês com o nome de Eriol, ou Ælfwine, que navega para o oeste e segue para Tol Eressëa, a Ilha Solitária, onde os elfos ainda moravam, e ouve deles as histórias antigas do Primeiras Idades. Esses contos são versões antigas e subdesenvolvidas daquelas que conhecemos do Silmarillion: Nomes são um pouco diferentes (Melkor é Melko, os Noldor são Gnomos (!)) E as palavras e a sintaxe das línguas élficas mudaram, mas as idéias básicas estão lá.

Então, para realmente voltar ao OP, em algum momento, Tolkien imaginou nosso presente - ou pelo menos uma versão dele - para ser o "futuro" da Terra Média. Mas este foi um conceito inicial que ele entreteve em sua juventude, e parece que no momento em que ele completou completamente, ele abandonou essa idéia.

1) Parece que estamos no futuro da Terra-média, já que Tolkien começou uma história envolvendo um homem de hoje viajando em tempo para Numenor no momento de sua queda.

2) Duas citações de O Retorno do Rei, capítulo 6 (Muitos Partments):

'Never is to long a word even for me,' said Treebeard. 'Not while your kingdoms last, you mean; but they will have to last long indeed to seem long to Ents.'

'The New Age begins,' said Gandalf, 'and in this age it may well prove that the kingdoms of Men shall outlast you, Fangorn my friend.'

e, um pouco depois:

[Treebeard: ] 'For the world is changing: I feel it in the water, I feel in the earth, and I smell it in the air. I do not think we shall meet again.'

And Celeborn said: 'I do not know, Eldest.' But Galadriel said: 'Not in Middle-earth, nor until the lands that lie under the wave are lifted up again. Then in the willow-meads of Tasarinan we may meet in the spring. Farewell!'

(outra referência a essa Segunda Profecia de Mandos )