If you're serious about creating a start-up business in the space sector then there's a lot of support and finance available to get you started, depending on which country you're in.
I'm in the UK so I can only give some pointers at a UK or Euro level but there are similar programmes running in many countries outside of Europe.
In the UK, the Technology Strategy Board (TSB) will fund projects that have the potential to generate revenue in the future. Perhaps you could work with a university to turn some research into a product, perhaps you could partner with one or more other SME's to form a consortium or perhaps you have the idea and skills already to put a proposal together yourself.
http://www.innovateuk.org/https://connect.innovateuk.org/web/spaceAs well as TSB there's the UK Space Agency and of course ESA who run an incubator on the Harwell Oxford campus, funding start-up's with bright new ideas. ESA provides funding across Europe so it isn't just a UK source of funding. If you're outside of Europe, perhaps you could partner with an SME or entrepreneur in Europe. NASA and other agencies provide similar funding opportunities.
Another good source of funding inside Europe is FP7 which provides billions of Euro's of funding across tech sectors including space.
http://cordis.europa.eu/fp7/home_en.htmlhttp://ec.europa.eu/research/participan ... ACE-2012-1There's a new space call coming up soon which will be a great opportunity for space start-up's and SME's.
Perhaps you could link your new idea to other sectors such as defence and get sponsorship from the Centre for Defence and Enterprise
http://www.science.mod.uk/engagement/enterprise.aspxThere are many other funding opportunities, the above are just coming from a UK perspective and is by no means comprehensive even for the UK.
As for ideas, the space sector is very broad with very many opportunities. CubeSat components and robotics are popular, as are propulsion and launch vehicles for small payloads. We are involved in some such launch vehicle concepts as well as services for commercial satellites.
That's actually another exciting area, especially in Europe. ESA is always looking for companies to come along with exciting new commercial uses for their data, perhaps in Earth observation or new services for their telecoms and media satellites. Essentially ESA has a lot of data and feeds which ultimately need to be commercialised so if you have a good idea for a product from their data and services then they are always keen to talk.
Data modelling and cloud computing and other technologies are all relevant to space, even mobile apps that helps to connect ESA with the tax payer.
Then of course materials, bio and medical products, small manufacturing units for space......there's just an endless list of possible ventures.
What I'd suggest is going to networking events, joining some good space business groups on LinkedIn and other specialist groups like the UK Space Entrepreneurs network.
As with any business, it's hard work and commitment. Don't expect to make a profit for the first year or two but with a good idea you can secure the funding to get through that period. If you're further along by that point it might be time to get funding from an angel investor and then after that venture capitalists.
Good luck with your business venture, it's an exciting time to be a space start-up.