I think this has been discussed before - and one of the main problems I see is a donation is not really a donation if you're expecting something in return. It's more a commercial contract - if feature X is completed, then payments from N people totalling Y are paid. But even as a commercial system where people lodge payments it sounds like it'd be extremely complicated to organise. If people donate money in to a holding account for a feature, what if nobody got round to implementing the feature? Would everyone get their money back? After how long? Would everyone be happy if that happened, eg. if somebody assumed they were definitely paying for a feature to happen? If people don't pay upfront and merely promise to pledge donations, what happens if the feature is done and some people don't follow through? I can't think of any good solutions to the problems of this style of payment model.
It's very kind of you to offer money for various features, but I see too many difficulties and would prefer donations to go the usual route of nothing-expected-in-return (but are still greatly appreciated and very motivating of course!). Also, something like an export-to-XBOX would be a huge project, lasting many months - probably best left to Construct 2 in the first place, but it's also difficult to imagine how donations could offset the time for that kind of project length as well.