first off, you are on an IFR clearance and not just a flight plan.
second off. if you are on a visual approach, with clearance for such, you are very likely to maintain visual contact with the airport and you have the airport in sight...unless it is a charted visual approach procedure and any of the landmarks noted on the chart is sufficient.
'it is quite possible to get a visual approach clearance to an airport that is not served by an instrument approach of any kind.
unless there is a visual missed approach procedure published, you as the pilot may GO AROUND if you want or need to...but since you are visual, maintain visual with the airport, if atc is available coordinate with them...if you are going into a non ATC airport, use your best judgement and come back and land.
IF you need to go IMC, back into the clouds for any reason, if there isn't a MSA published, you have to use YOUR knowledge to comply with terrain clearance...re contact ATC advise them you are IMC and unable to land off the visual approach and they will work something out.
Generally speaking there is adequate clearance from other IFR traffic maintained until you are on the ground and ATC cancels your IFR clearance, or until you cancel.
as a reminder, 2000' clearance in mountainious terrain, 1000' in non mountaionous.
OF course if you have been ''cheating'' and said you had the airport in sight and NO ONE ELSE EVER REPORTS IT IN SIGHT, there might be a word or two with the FAA or authority..
In actuality, either VMC or IMC , after you notify ATC, they will tell you what to do...BUT IF YOU CAN'T TALK TO THEM...You better be ready with knowledge and action to avoid terrain and keep yourself and other planes safe.