If you own euros and you want to be long on EURUSD, wouldn't it be more logical and with less fees to open a position on USDEUR and do a margin deposit directly in euros instead of convert the euros in dollars, then do the opposite way after closing the position ?
Your question is about futures, which trade on a Futures Exchange.
The only future contract that I know that meets your requirement is the "Euro FX" contract that trades on the CME (Chicago Mercantile Exchange) under the ticker 6E. It is quoted in EURUSD, one contract calls for the delivery of EUR 125,000 and requires a margin deposit in USD.
Certainly for some people it might be more convenient to trade a future quoted in USDEUR and with a margin in EUR, but as far as I know no exchange offers one. Probably it would be difficult for regulatory reasons for a US based Exchange to offer such a product (in US, margins are always in USD AFAIK). But a European futures exchange could certainly offer one. Perhaps you could write to the marketing department of Eurex and suggest that they introduce such a product. (Or perhaps such a contract already exists and I haven't heard about it).