So the answer you got from Kazzeon is already that what you searched for probably. ^^
I am aware that you want to stress that you will become a father next year, however, I would like to give you another submission for your letter. How would it be to say something like: “Before becoming father next year, should we not travel to Japan together?”. It sounds a tad fancier connecting these two rather short sentences.
来年、父になる前、一緒に日本に旅行しに行きませんか。
I have to admit that I am by no means fluent in Japanese but I think asking this question rather than uttering a desire seems more natural and polite. Also the context should be taken into account. It is a letter by you to your father so you do not need 私 since it is already in the context nor a 私の父. However, if you want to stress that you will become what your father once did it makes much sense implementing something like 私の父. Furthermore you can alter 行きませんか to 行かないの when you are at a familiar level with your father as I suppose. It is all up to you of course and I hope I could give you another option to express your thought.
I am by no means even close to fluent, but I like 行きませんか as well. If I were constructing the sentence, I’d probably also have dropped the 旅行 part. It seems strange to me to use 行 twice. Repeated disclaimer, I’m not fluent. But in the classes I’ve taken, I think we’d do その前に一緒に日本へ行きませんか。
Direct translation:
Before the child is born and I become a father next year, I am thinking it’d be enjoyable to travel with (you, my honorific) father, and so, please let me know a time that’d be convenient for you.
Meaning translation:
before I become a father next year, I think it’d be enjoyable to travel with you. However, when would it be convenient for you?
if 知らせてください sounds formal, 言ってください will work as well.
I’m curious if you’re writing a novel, or, if you’re biography is as interesting as I imagine it to be…
I spoke with a native about your sentence because I did not know what 楽しんだらと思っています would mean and he/she said also the following:
For everyone talking about blackblaze5’s sentence. In the context, (1) 楽しんだら is unnatural. So please use potential and assume form. (2) ご都合 is wrong word. Don’t use ご for speaker’s things/actionss. And 都合 means schedules/plans/time. (3) Perhaps listeners can understand what means 知らせて. But it’s an unnatural word. So please use another translation for “tell”. Thanks
I posted it not to insult but to inform you/us of course.